Merge branch 'ef-bvm-saturday': the Office of Our Lady on Saturday

Builds RG 91 entry 27 under RG 78 -- 'In sabbatis, in quibus occurrit
Officium de feria IV classis, fit de sancta Maria in sabbato' -- a rank
test, not a season list. band had been assigning entry 27 312966 times
across the domain while the office was never constructed, so an
otherwise-unoccupied Saturday got a bare ferial slug and its season's
colour.

75863 days change, in two shapes: colour (green or violet to white) on
74642, and colour plus a dropped commemoration on 1221. The 312966
reconciles to the day across four buckets, 210808 of them impeded by a
sanctoral winner.

The first version introduced a live RG 112(d) violation on every
Saturday falling on 16 July: the Office of Our Lady observed,
commemorating Our Lady. Both photographic scans print a rubric at that
Mass for exactly this collision -- 'Missa dici potest aut de sancta
Maria in sabbato, aut propria' -- absent from the electronic
transcription, and cited nowhere. It was missed because the check was a
grep for subject = Bvm, and Mt Carmel is tagged Saint: the rubric's
criterion is whose intercession the oration implores, not colitur's own
tag.

The I-V Mass numbering is a propers rotation and stays unbuilt;
citations remain empty on all 3073916 days.

Recorded rather than glossed: the new rule has no comparison-layer
witness at all, and lectio's fixture prints a Mt Carmel commemoration on
8 in-window dates where colitur now prints none -- a real divergence
that falls through a gap in what the comparators examine. Time after
Epiphany's 5015 office days have zero oracle witnesses and rest on the
scan alone.

5 files changed, 508 insertions, 43 deletions

diff --git a/test/test_differential.ml b/test/test_differential.ml
index bea1bf4..5093484 100644
--- a/test/test_differential.ml
+++ b/test/test_differential.ml
@@ -138,6 +138,7 @@ module Slug = Colitur_kernel.Slug
 module Date = Colitur_kernel.Date
 module Cel = Colitur_kernel.Celebration
 module Colour = Colitur_kernel.Colour
+module Subject = Colitur_kernel.Subject
 module V = Rite_ef.Vocab_ef
 
 (* Same relative paths test_rite_ef.ml/test_sanctoral_ef.ml use: dune test
@@ -216,6 +217,14 @@ type row = {
   slug : string;
   rank : string;
   colour : string;
+  (* Fix round 1 (coordinator finding F3): ONLY ever populated for colitur's
+     own row (see [colitur_rows_2005_2050] below) -- lectio's own fixture
+     format has no subject column at all, so [row_of_line] (the lectio-side
+     parser) sets this to "-", a placeholder never read on that side. Every
+     Layer C predicate that reads [subject] must therefore read it off the
+     COLITUR row [c], never [l] -- the same asymmetry [row]'s other fields
+     do not have, called out here rather than left implicit. *)
+  subject : string;
 }
 
 let read_lines path =
@@ -232,7 +241,7 @@ let read_lines path =
 let row_of_line line =
   match String.split_on_char ' ' line with
   | date :: weekday :: season :: week :: slug :: rank :: colour :: _others ->
-      { date; weekday; season; week; slug; rank; colour }
+      { date; weekday; season; week; slug; rank; colour; subject = "-" }
   | _ -> Alcotest.failf "malformed fixture line (fewer than 7 fields): %S" line
 
 let lectio_rows () = List.map row_of_line (read_lines fixture_path)
@@ -271,7 +280,8 @@ let colitur_rows_2005_2050 () =
               week;
               slug = Slug.to_string cel.Cel.slug;
               rank = V.rank_to_string cel.Cel.rank;
-              colour = Colour.to_string cel.Cel.colour
+              colour = Colour.to_string cel.Cel.colour;
+              subject = Subject.to_string cel.Cel.subject
             }
             :: !rows
       | None -> Alcotest.failf "internal error: no resolved day for %s" (Date.to_iso8601 !d));
@@ -516,6 +526,44 @@ let holy_name_fallback_dates_2005_2050 =
     "2034-01-02"; "2035-01-02"; "2036-01-02"; "2040-01-02"; "2041-01-02"; "2045-01-02";
     "2046-01-02"; "2047-01-02" ]
 
+(* C17 -- ef-bvm-saturday task: RG 91 entry 27 ("Officium sanctae Mariae in
+   sabbato" -- see data/ef/expected-divergences.sexp's own C17 note for the
+   full RG 78/RG 120(b) citation). lectio builds no equivalent office at all
+   (checked directly against its own tridentine-calendar.ini/generator, the
+   same "genuine upstream gap, not a colitur bootstrap miss" shape C15/C16
+   already document for their own gaps) -- it shows the ordinary season
+   colour on every otherwise-unoccupied Class4 Saturday where colitur's own
+   RG 78 fix now shows white. Season, slug and rank all still agree --
+   temporal_ef.ml's own [bvm_saturday_names] citation ("Slug" paragraph)
+   deliberately REUSES the ordinary ferial slug rather than minting a new
+   one, precisely so this predicate's diff set stays [Colour_f] alone, never
+   [Slug_f] too. Gated on colitur's own [rank]/[weekday]/[colour] rather
+   than a date list, the same shape M2 (data/ef/expected-divergences-
+   missalemeum.sexp) already uses for the identical reason: this
+   population is large (a large fraction of all Saturdays domain-wide) and
+   entirely formulaic (RG 78's own condition, "otherwise unoccupied
+   IV-class Saturday", reduces exactly to this triple), not a short,
+   individually-interesting list of dates the way C14/C15/C16 above are. *)
+let is_bvm_saturday_row (c : row) diffs =
+  (* Fix round 1 (coordinator finding F3): [c.subject] added -- the ONLY
+     field in this predicate that pins WHICH celebration is observed, not
+     merely its shape. Every other Layer C entry pins a colitur slug (C1's
+     own [jan_6_13_slug], C6's [nativity_octave_day_slugs], C8/C14/C15/C16's
+     own literal slug checks); this entry could not, because the office
+     deliberately REUSES the ordinary ferial slug (Rite_ef.Temporal_ef's own
+     [bvm_saturday_names] citation, "Slug" paragraph) -- there is no fixed
+     slug string to pin. [subject = "bvm"] is the field that DOES uniquely
+     identify the office (set nowhere else the differential's own
+     [colitur_rows_2005_2050] can produce a Saturday/Class4/white
+     combination for), closing the gap a shape-only predicate left open:
+     without this conjunct, a FUTURE bug that made some OTHER white,
+     Class4, Saturday candidate exist (Christmastide/Paschaltide, where
+     [season_colour] is already white, so a real bug there could slip
+     through unnoticed by colour alone) would be silently absorbed here
+     too. *)
+  diffs = [ Colour_f ] && String.equal c.weekday "saturday" && String.equal c.rank "class-4"
+  && String.equal c.colour "white" && String.equal c.subject "bvm"
+
 (* [layer_c_reason l c diffs] returns the [data/ef/expected-divergences.sexp]
    [id] this row-pair's remaining (post Layer A/B) diff set belongs to, or
    [None] if nothing here explains it (a genuine, uncovered failure). *)
@@ -571,6 +619,7 @@ let layer_c_reason (l : row) (c : row) diffs =
     && subset diffs [ Slug_f; Rank ]
     && String.equal c.slug "ef-holy-name"
   then Some "C16"
+  else if is_bvm_saturday_row c diffs then Some "C17"
   else None
 
 (* ---------------------------------------------------------------------- *)
diff --git a/test/test_golden.ml b/test/test_golden.ml
index 594ea6b..fdfc01d 100644
--- a/test/test_golden.ml
+++ b/test/test_golden.ml
@@ -851,8 +851,22 @@ let test_iii_class_feast_wins_class4_feria () =
     "2008-04-05 saturday season=paschaltide week=2 slug=vincent-ferrer rank=class-3 colour=white subject=saint name_la=- comms=[] in=- \
      out=[]";
   let d4 = fetch 2038 3 6 in
+  (* ef-bvm-saturday task: 6 March 2038 is a SATURDAY, so the displaced
+     temporal candidate is now RG 78's own BVM Saturday office
+     ({!Rite_ef.Temporal_ef}'s own [bvm_saturday_names] citation) rather than
+     a bare ferial -- but per that citation's own "Slug" paragraph the SLUG
+     is deliberately left unchanged (reused, not replaced), precisely so
+     this test's own [omitted_has] probe still needs no update: the displaced
+     candidate is still named "ef-septuagesima-2-saturday", only its
+     colour/subject/name would have differed had it won. RG 91 entry 24
+     (Sts Perpetua and Felicity) still outranks it either way -- entry 24 <
+     entry 27, both above entry 28 -- so the disposition/win outcome this
+     test proves is genuinely unchanged by this task, now more precisely
+     entry 24 > entry 27 rather than > 28 for this one date (a Saturday), not
+     that the test itself needed touching. *)
   Alcotest.(check bool)
-    "2038-03-06: the displaced class-4 Septuagesima feria is in [omitted] (RG26), proving RG91 entry24>28 fired"
+    "2038-03-06: the displaced class-4 feria (RG78's own BVM-Saturday office, \
+     for this Saturday) is in [omitted] (RG26), proving RG91 entry24>27 fired"
     true
     (omitted_has d4 "ef-septuagesima-2-saturday");
   Alcotest.(check string)
@@ -869,6 +883,81 @@ let test_iii_class_feast_wins_class4_feria () =
     "2038-03-09 tuesday season=septuagesima week=3 slug=frances-rome rank=class-3 colour=white subject=saint name_la=- comms=[] in=- \
      out=[]"
 
+(* ------------------------------------------------------------------ *)
+(* RG 91 entry 27 / RG 78-79 (Caput IX, "De sancta Maria in sabbato") --
+   ef-bvm-saturday task, Rite_ef.Temporal_ef's own [bvm_saturday_names]
+   citation has the full primary-source argument. Two real-data pins, the
+   task brief's own worked example and its mirror-image negative case,
+   matching this file's own header rule: EVERY literal below was checked by
+   hand against `colitur day 2026` before being typed in here (this task's
+   own report has the full session log), never copied from a run and
+   rationalised after. *)
+
+(* The task brief's own worked example, checked against the oracle
+   (missalemeum) directly: "2026-08-01|4|w|V Mass of the B. V. M. -- Salve,
+   Sancta Parens|Saturday_after_IX_Sunday_after_Pentecost|Holy Machabees".
+   colitur's own rank (4) and colour (white) now match; the oracle's own
+   numbered-Mass title (RG 309(a)'s own "iuxta temporum diversitatem", not
+   modelled here -- {!bvm_saturday_names}'s own "Slug" paragraph) and the
+   observed day's own English identity remain outside what this comparator
+   can see (M18's own limit) -- this pin instead proves what colitur DOES
+   compute: white/class-4/subject Bvm/the Latin title, and Holy Machabees
+   (real data/ef/sanctoral.sexp entry, Commemoration_only, Class3, 1 August)
+   still admitted as an ORDINARY commemoration alongside it, matching the
+   oracle's own commemoration list exactly. *)
+let test_bvm_saturday_2026 () =
+  check ~msg:"2026-08-01: the BVM Saturday office observed, white overriding Time after Pentecost's green, \
+              Holy Machabees commemorated -- matches the task brief's own oracle example"
+    2026 8 1
+    "2026-08-01 saturday season=time-after-pentecost week=9 slug=ef-time-after-pentecost-9-saturday \
+     rank=class-4 colour=white subject=bvm name_la=Officium sanctae Mariae in sabbato \
+     comms=[holy-machabees:ordinary] in=- out=[]"
+
+(* The mirror-image case: RG 78's own protasis is FALSE here (a real feast
+   wins the Saturday outright), so the BVM-Saturday-shaped temporal
+   candidate must lose and be omitted under RG 26, exactly like the plain
+   ferial candidate it replaces always was -- no commemoration at all,
+   proving [band]/[disposition] need no special case for this office (the
+   design claim the task report makes explicitly). ALSO the one live data
+   witness where the winning candidate itself carries [subject = Bvm]
+   (data/ef/adjustments.sexp's own `Edit most-holy-name-of-mary
+   ((Set_subject Bvm))`) -- proving no Bvm-vs-Bvm confusion with a
+   RG112(a)-style subject rule: [omitted_has] below confirms the loser is
+   genuinely reported omitted (RG26 fires on [rank = Class4] alone, checked
+   ahead of any subject-based branch in {!Rite_ef.Precedence_ef.disposition}),
+   not silently swallowed by a same-subject exclusion that does not exist
+   for [Bvm] anywhere in this codebase. *)
+let test_bvm_saturday_loses_to_most_holy_name_of_mary_2026 () =
+  let d = fetch 2026 9 12 in
+  Alcotest.(check bool)
+    "2026-09-12: the displaced BVM-Saturday-shaped candidate is in [omitted] (RG26), not silently absent"
+    true
+    (omitted_has d "ef-time-after-pentecost-15-saturday");
+  Alcotest.(check string)
+    "2026-09-12: Most Holy Name of Mary (III class, subject Bvm, data/ef/adjustments.sexp) outranks the \
+     class-4 BVM-Saturday candidate; no commemoration at all"
+    "2026-09-12 saturday season=time-after-pentecost week=15 slug=most-holy-name-of-mary rank=class-3 \
+     colour=white subject=bvm name_la=- comms=[] in=- out=[]"
+    (describe d)
+
+(* RG112(d), fix round 1 (coordinator finding F1): PE.marian_slugs's own
+   citation (precedence_ef.ml) has the full RG112(d) argument, including
+   the 16 July "Missa dici potest aut ... aut ..." rubric found on both
+   photographic scans. 16 July 2033 is a real otherwise-unoccupied
+   Saturday; before this fix, [comms] here read
+   "[our-lady-of-mt-carmel:ordinary]" (confirmed against real pre-fix
+   `colitur day 2033` output). *)
+let test_bvm_saturday_excludes_mt_carmel_2033 () =
+  let d = fetch 2033 7 16 in
+  Alcotest.(check bool)
+    "2033-07-16: Mt Carmel is in [omitted] (RG112(d)), not silently dropped from the record" true
+    (omitted_has d "our-lady-of-mt-carmel");
+  Alcotest.(check string)
+    "2033-07-16: the BVM Saturday office observed, white; NO commemoration of Mt Carmel (RG112(d))"
+    "2033-07-16 saturday season=time-after-pentecost week=5 slug=ef-time-after-pentecost-5-saturday \
+     rank=class-4 colour=white subject=bvm name_la=Officium sanctae Mariae in sabbato comms=[] in=- out=[]"
+    (describe d)
+
 let suite =
   ( "golden pins (known-tricky years)",
     [ Alcotest.test_case "Easter extreme: 1598 earliest (22 Mar, Gauss-verified)" `Quick
@@ -925,5 +1014,17 @@ let suite =
       Alcotest.test_case
         "ef-rebootstrap: RG91 entry24 (III-class feast) outranks entry28 (class-4 feria) -- the six dates \
          deferred since 2038's own Easter-extreme pin (2008-04-02/04/05, 2038-03-06/08/09)"
-        `Quick test_iii_class_feast_wins_class4_feria
+        `Quick test_iii_class_feast_wins_class4_feria;
+      Alcotest.test_case
+        "RG91 e27/RG78: the BVM Saturday office wins, Holy Machabees commemorated -- matches the task \
+         brief's own oracle example (2026-08-01)"
+        `Quick test_bvm_saturday_2026;
+      Alcotest.test_case
+        "RG91 e27/RG78/RG26: the BVM Saturday office loses to Most Holy Name of Mary (subject Bvm too) \
+         and is omitted, no confusion (2026-09-12)"
+        `Quick test_bvm_saturday_loses_to_most_holy_name_of_mary_2026;
+      Alcotest.test_case
+        "RG112(d), fix round 1 (F1): the BVM Saturday office excludes Mt Carmel's own commemoration \
+         (2033-07-16)"
+        `Quick test_bvm_saturday_excludes_mt_carmel_2033
     ] )
diff --git a/test/test_oracle.ml b/test/test_oracle.ml
index 58ceb26..c0badfe 100644
--- a/test/test_oracle.ml
+++ b/test/test_oracle.ml
@@ -475,25 +475,30 @@ let contains_substring s ~needle =
    own "in dominica quavis" (ANY Sunday, unqualified) -- verdict colitur. *)
 let m1_dates = [ "2026-06-28"; "2026-08-09" ]
 
-(* M2 -- RG 91 entry 27 ("Officium sanctae Mariae in sabbato"): every
-   otherwise-unoccupied IV-class Saturday should carry the votive Office of
-   the BVM (white; missalemeum's own title cycles "I".."V Mass of the
-   B. V. M. - Salve, Sancta Parens"), a genuinely DISTINCT office from an
-   ordinary green/violet feria. temporal_ef.ml's [band] already carries an
-   entry-27 comment acknowledging this row of RG 91 exists, but nothing in
-   [Temporal_ef.temporal]'s actual CONSTRUCTION builds this office -- an
-   unimpeded Time-after-Epiphany/-Pentecost Saturday still gets the bare
-   ferial slug and [season_colour]'s green. A genuine, primary-attested
-   FEATURE GAP (not a citation dispute) -- verdict missalemeum: colitur is
-   short a whole office here, not merely differently-opinioned. Registered
-   as an open item (register §6) rather than built in this task: it needs
-   its own proper texts/citations, the same scope line "Major Litanies"
-   (M5 below) already sits on. Matched on the oracle's own title substring,
-   not a hardcoded date list -- the title IS the distinguishing evidence,
-   and every Paschaltide occurrence (where colitur's season colour is
-   already white, RG 119) never reaches this predicate at all, since its
-   diffs are already empty before [layer_m_reason] is ever called. *)
-let bvm_saturday_title = "Salve, Sancta Parens"
+(* M2 -- CLOSED, REMOVED (ef-bvm-saturday task), same shape as M12/M17's own
+   removal elsewhere in this suite (not re-adjudicated to a different
+   verdict, gone because the underlying divergence stopped occurring): RG 91
+   entry 27 ("Officium sanctae Mariae in sabbato") is now built
+   (Rite_ef.Temporal_ef's own [bvm_saturday_names] citation has the full RG
+   78/RG 120(b) argument) -- white, unconditionally, on every otherwise-
+   unoccupied IV-class Saturday, so [Colour_f] no longer appears in this
+   predicate's own diff set on ANY of these 22 dates (checked: 0 of them
+   retain it). What is left on every one of them is [Observed_identity_
+   unresolved] ALONE -- colitur's own BVM-Saturday candidate is
+   temporal-origin and deliberately carries no ENGLISH name (Latin only,
+   the same zero-circularity discipline Holy Family/Holy Name/the Sacred
+   Triduum already established) -- which is EXACTLY [M18]'s own shape
+   ([diffs = [ Observed_identity_unresolved ]], nothing else disagreeing),
+   not a distinct citation any more. Kept as a separate id here would have
+   been the same "count proving cardinality where identity was required"
+   trap this project's own review process watches for: [subset] admits a
+   SMALLER diff set than the one named, so this branch would have kept
+   silently absorbing these 22 rows under a stale "colour differs" citation
+   even though that citation's own reason no longer fires -- checked
+   directly, not assumed: removing this branch and re-running moves the
+   count from M2's own former 22 to M18's own count rising by exactly 22
+   (373 -> 395, see M18's own note below), with the "no unexplained
+   differences" test staying green throughout. *)
 
 (* M3 -- RG 87 (Minor Litanies/Rogations): the SAME gap the lectio
    differential's own C8 already names (data/ef/expected-divergences.sexp)
@@ -822,7 +827,12 @@ let m19_commemoration_matches (c : colitur_row) =
   | [ (slug, _, _, _) ] -> String.equal slug "commemoration-of-st-peter"
   | _ -> false
 
-let layer_m_reason (c : colitur_row) (o : oracle_row) diffs =
+(* [_o] (the oracle's own row) is unused now that M2 -- its one reader, via
+   [o.o_title] -- is closed/removed (see the comment above [m1_dates]):
+   every remaining predicate below reads only [c]/[diffs]. Kept as a named,
+   typed parameter (not dropped from the signature) so a FUTURE oracle-title
+   predicate has somewhere to plug back in without a signature change. *)
+let layer_m_reason (c : colitur_row) (_o : oracle_row) diffs =
   if diffs = [] then None
   (* M1's own subset widened (this task): colitur's observed day on both
      dates is TEMPORAL-origin (the Sunday itself, RG33's own point -- the
@@ -832,17 +842,12 @@ let layer_m_reason (c : colitur_row) (o : oracle_row) diffs =
   else if List.mem c.c_date m1_dates
           && subset diffs [ Rank; Colour_f; Comm_presence; Observed_identity_unresolved ]
   then Some "M1"
-  (* M2's own subset widened (this task): every BVM-Saturday date's observed
-     celebration is temporal-origin (the ordinary Saturday feria colitur
-     computes, not the distinct Office of the BVM missalemeum's title
-     names -- M2's own gap), so [Observed_identity_unresolved] now fires
-     alongside [Colour_f] where it used to fire alone, AND alone on its own
-     on every Paschaltide occurrence M2's own comment already says never
-     reached this predicate before (colour already agrees there; only the
-     new axis has anything left to say). *)
-  else if contains_substring o.o_title ~needle:bvm_saturday_title
-          && subset diffs [ Colour_f; Observed_identity_unresolved ]
-  then Some "M2"
+  (* M2 -- CLOSED, REMOVED (ef-bvm-saturday task): see this file's own M2
+     note above (near [m1_dates]) for the full account -- the RG 91 entry 27
+     office is now built, so every one of these 22 dates now falls through
+     to [M18]'s own generic predicate below instead ([diffs = [
+     Observed_identity_unresolved ]] exactly), which is what actually
+     catches them now. *)
   (* M3's own subset widened (this task): the Rogation Monday feria colitur
      observes is temporal-origin, same root cause as M1/M2 above. *)
   else if List.mem c.c_date m3_dates && subset diffs [ Colour_f; Observed_identity_unresolved ] then
diff --git a/test/test_precedence_ef.ml b/test/test_precedence_ef.ml
index ed9a933..6f8b5b5 100644
--- a/test/test_precedence_ef.ml
+++ b/test/test_precedence_ef.ml
@@ -291,10 +291,21 @@ let cases =
       cand ~origin:P.Sanctoral ~rank:V.Class3 ~layer:PE.universal_layer "vigil-of-st-lawrence",
       260 );
     (* Entry 27 -- RG 91 entry 27 (§4): an otherwise-unoccupied IV-class
-       Saturday. *)
-    ( "27 Office of the BVM on Saturday", off 62,
-      cand ~rank:V.Class4 "ef-time-after-pentecost-1-sat",
-      270 );
+       Saturday. ef-bvm-saturday task: sourced from [Temporal_ef.temporal]'s
+       own real output (see [of_temporal], the same review-finding-3
+       discipline entry 18's/21's own rows above use), not a hand-typed
+       slug/rank pair -- this row used to be the one place in this whole
+       suite where [band]'s entry-27 branch was exercised by a value nothing
+       in [Temporal_ef.temporal] could actually produce (the office itself
+       was unbuilt): [band] read [rank = Class4 && weekday = Sat]
+       correctly, but nothing proved [temporal] ever handed it a candidate
+       shaped that way for the RIGHT reason. Now it does: [of_temporal (off
+       62)] is 6 June 2026, a real otherwise-contested Time-after-Pentecost
+       Saturday (Norbert, Class3, wins the DAY outright in the real
+       pipeline -- irrelevant here, since this row tests [band] on the
+       TEMPORAL CANDIDATE alone, which [Temporal_ef.temporal] builds as the
+       BVM Saturday office regardless of what wins). *)
+    ( "27 Office of the BVM on Saturday (from Temporal_ef.temporal)", off 62, of_temporal (off 62), 270 );
     (* Entry 28 -- RG 91 entry 28 (§4): the unqualified IV-class catch-all. *)
     ("28 IV-class feria", off 65, cand ~rank:V.Class4 "ef-time-after-pentecost-1-tue", 280);
     (* Not an RG 91 row at all: a I-class candidate marked as a vigil, which
@@ -794,6 +805,52 @@ let disposition_cases =
         "conversion-of-st-paul",
       cand ~origin:P.Sanctoral ~rank:V.Class2 ~subject:Sub.Lord ~layer:PE.universal_layer
         "commemoration-of-the-baptism-of-the-lord",
+      "Commemorate(Ordinary)" );
+    (* RG112(d), fix round 1 (coordinator finding F1) -- PE.marian_slugs's
+       own citation has the full argument (both scans, including the 16
+       July "Missa dici potest aut ... aut ..." rubric). [of_temporal] on a
+       REAL 16-July-Saturday (2033) sources the winner from
+       Temporal_ef.temporal's own real output, the same review-finding-3
+       discipline this file's Ember/vigil rows already use -- not a
+       hand-typed slug/subject pair that could silently drift from what the
+       office actually builds. The loser is the real data/ef/sanctoral.sexp
+       entry, [Commemoration_only], the exact shape that made this branch
+       need to be checked BEFORE the Commemoration_only catch-all. *)
+    ( "RG112(d): the BVM Saturday Office excludes a commemoration invoking \
+       the SAME BVM's intercession -- Omit, not Commemorate (Mt Carmel, \
+       16 July 2033, a real otherwise-unoccupied Saturday)",
+      of_temporal (mk 2033 7 16),
+      cand ~origin:P.Sanctoral ~rank:V.Class3 ~status:Cel.Commemoration_only ~subject:Sub.Saint
+        ~layer:PE.universal_layer "our-lady-of-mt-carmel",
+      "Omit" );
+    (* Conjunct 1/2, LOSER identity: an ORDINARY Commemoration_only saint
+       NOT on PE.marian_slugs must still reach the ordinary commemorate
+       flow even when the winner is the BVM Saturday Office -- proves
+       RG112(d) excludes a specific same-BVM collision, not every
+       commemoration the office happens to outrank (the real live shape,
+       1 August, Holy Machabees, PE.marian_slugs's own comment). *)
+    ( "RG112(d) does NOT fire when the LOSER is not a Marian commemoration \
+       -- an ordinary saint stays Commemorate(Ordinary) even under the BVM \
+       Saturday Office (1 August 2026, Holy Machabees)",
+      of_temporal (mk 2026 8 1),
+      cand ~origin:P.Sanctoral ~rank:V.Class3 ~status:Cel.Commemoration_only ~subject:Sub.Saint
+        ~layer:PE.universal_layer "holy-machabees",
+      "Commemorate(Ordinary)" );
+    (* Conjunct 2/2, WINNER identity: an ordinary (non-Bvm, non-Marian-
+       titled) winner must not exclude Mt Carmel either -- RG112(d) is
+       keyed on the WINNER also being "de B. Maria Virg.", not merely on
+       the loser's own identity. SYNTHETIC: no live date has an ordinary
+       Class3 saint winning outright while Mt Carmel also loses that same
+       day (Mt Carmel's own fixed date, 16 July, has no other real
+       sanctoral entry to construct this from) -- proves the conjunct is
+       real, not merely untested, the same "SYNTHETIC" discipline the
+       RG112(a) rows above already use for their own second conjunct. *)
+    ( "SYNTHETIC: RG112(d) does NOT fire when only the LOSER is Marian -- \
+       an ordinary winner leaves Mt Carmel Commemorate(Ordinary)",
+      cand ~origin:P.Sanctoral ~rank:V.Class3 ~subject:Sub.Saint ~layer:PE.universal_layer
+        "ef-synthetic-ordinary-winner",
+      cand ~origin:P.Sanctoral ~rank:V.Class3 ~status:Cel.Commemoration_only ~subject:Sub.Saint
+        ~layer:PE.universal_layer "our-lady-of-mt-carmel",
       "Commemorate(Ordinary)" )
   ]
 
@@ -1421,6 +1478,107 @@ let test_ii_class_sunday_privileged_witness_admitted_end_to_end () =
     [ ("ef-some-saint", "omitted: admission limit reached") ]
     (List.map (fun (c, reason) -> (S.to_string c.P.cel.Cel.slug, reason)) resolution.P.omitted)
 
+(* RG 91 entry 27 / RG 78-79 (Caput IX, "De sancta Maria in sabbato") reached
+   end-to-end through the REAL pipeline -- this is THE hazard this task's own
+   brief names explicitly: {!PE.band}'s entry-27 branch has been reachable
+   only in isolation (the table-driven row above) since it was written, never
+   through a real [Temporal_ef.temporal] candidate winning an actual day, so
+   this test is checked for whether it exercises entry 27 for the RIGHT
+   reason -- proved by the mutation test recorded in the task report
+   (reverting temporal_ef.ml's own [is_bvm_saturday] branches back to the
+   generic ferial fallback reddens THIS test, not merely the [band]-table
+   row, which would still pass unchanged since it is sourced from real
+   [T.temporal] output either way).
+
+   1 August 2026, matching the task brief's own worked example against the
+   oracle exactly: a Time-after-Pentecost Saturday, otherwise unoccupied but
+   for Holy Machabees (Commemoration_only, Class3, real data/ef/sanctoral.sexp
+   entry, 1 August) -- the office wins outright (band 270, ahead of Holy
+   Machabees's own [unclassified]) and the Commemoration_only loser still
+   rides along as an ordinary commemoration, the SAME shape the oracle's own
+   row for this date shows ("V Mass of the B. V. M. -- Salve, Sancta Parens",
+   commemorating Holy Machabees). *)
+let test_bvm_saturday_wins_and_commemorates_end_to_end () =
+  let date = mk 2026 8 1 in
+  let day_ctx = ctx date in
+  let bvm_saturday = of_temporal date in
+  let holy_machabees =
+    cand ~origin:P.Sanctoral ~rank:V.Class3 ~status:Cel.Commemoration_only ~layer:PE.universal_layer
+      "holy-machabees"
+  in
+  let resolution = P.resolve real_rules day_ctx ~temporal:bvm_saturday ~sanctoral:[ holy_machabees ] in
+  Alcotest.(check string) "the BVM Saturday office is observed, not Holy Machabees"
+    "ef-time-after-pentecost-9-saturday" (S.to_string resolution.P.observed.P.cel.Cel.slug);
+  Alcotest.(check string) "observed rank is class-4, unconditional" "class-4"
+    (V.rank_to_string resolution.P.observed.P.cel.Cel.rank);
+  Alcotest.(check string) "observed colour is white, overriding the season's own green" "white"
+    (Col.to_string resolution.P.observed.P.cel.Cel.colour);
+  Alcotest.(check bool) "observed subject is Bvm" true (resolution.P.observed.P.cel.Cel.subject = Sub.Bvm);
+  Alcotest.(check (list (pair string string))) "Holy Machabees still rides along as an ordinary commemoration"
+    [ ("holy-machabees", "Ordinary") ]
+    (List.map
+       (fun (c, p) -> (S.to_string c.P.cel.Cel.slug, match p with P.Privileged -> "Privileged" | P.Ordinary -> "Ordinary"))
+       resolution.P.commemorations);
+  Alcotest.(check int) "nothing omitted" 0 (List.length resolution.P.omitted)
+
+(* The other half of the SAME hazard: a real Saturday where RG 78's own
+   protasis is FALSE (a genuine sanctoral feast wins the day outright), so
+   the BVM-Saturday-shaped temporal candidate must lose, and be OMITTED under
+   RG 26 (a temporal Class4 non-vigil loser is never commemorated) exactly
+   like the plain ferial candidate it replaces always was -- proving [band]/
+   [disposition] need no special case for this office at all, the design
+   claim the task report makes explicitly. 12 September 2026 is a Saturday
+   where `most-holy-name-of-mary` (data/ef/adjustments.sexp's own [Set_subject
+   Bvm] entry, Class3 Feast) wins outright -- ALSO tagged [subject = Bvm],
+   the one live data witness that could in principle confuse a
+   subject-keyed disposition rule with RG 26's own rank-keyed one: this test
+   proves they do NOT collide (RG 26 fires first, unconditionally, on the
+   loser's own [rank = Class4], never reaching a subject comparison at all). *)
+let test_bvm_saturday_loses_to_a_real_feast_end_to_end () =
+  let date = mk 2026 9 12 in
+  let day_ctx = ctx date in
+  let bvm_saturday = of_temporal date in
+  let name_of_mary =
+    cand ~origin:P.Sanctoral ~rank:V.Class3 ~subject:Sub.Bvm ~layer:PE.universal_layer
+      "most-holy-name-of-mary"
+  in
+  let resolution = P.resolve real_rules day_ctx ~temporal:bvm_saturday ~sanctoral:[ name_of_mary ] in
+  Alcotest.(check string) "Most Holy Name of Mary is observed, not the BVM Saturday office"
+    "most-holy-name-of-mary" (S.to_string resolution.P.observed.P.cel.Cel.slug);
+  Alcotest.(check (list string)) "nothing is commemorated" []
+    (List.map (fun (c, _) -> S.to_string c.P.cel.Cel.slug) resolution.P.commemorations);
+  Alcotest.(check (list (pair string string)))
+    "the BVM-Saturday-shaped temporal candidate is omitted under RG26, not commemorated -- no Bvm-vs-Bvm \
+     confusion with RG112(a)-style subject logic"
+    [ ("ef-time-after-pentecost-15-saturday", "omitted: yielded to a higher day") ]
+    (List.map (fun (c, reason) -> (S.to_string c.P.cel.Cel.slug, reason)) resolution.P.omitted)
+
+(* RG112(d), fix round 1 (coordinator finding F1) -- PE.marian_slugs's own
+   citation has the full argument. End-to-end through the REAL pipeline
+   (PE.band, PE.disposition AND PE.admit together): 16 July 2033, a real
+   otherwise-unoccupied Saturday, with the real data/ef/sanctoral.sexp
+   [our-lady-of-mt-carmel] entry as the day's only competing candidate.
+   Before this fix, [resolution.P.commemorations] held
+   [our-lady-of-mt-carmel:Ordinary] (confirmed live against real 2005/2033
+   `colitur day` output before this fix existed); after, it is empty and
+   Mt Carmel is reported omitted, not silently dropped. *)
+let test_bvm_saturday_excludes_mt_carmel_end_to_end () =
+  let date = mk 2033 7 16 in
+  let day_ctx = ctx date in
+  let bvm_saturday = of_temporal date in
+  let mt_carmel =
+    cand ~origin:P.Sanctoral ~rank:V.Class3 ~status:Cel.Commemoration_only ~subject:Sub.Saint
+      ~layer:PE.universal_layer "our-lady-of-mt-carmel"
+  in
+  let resolution = P.resolve real_rules day_ctx ~temporal:bvm_saturday ~sanctoral:[ mt_carmel ] in
+  Alcotest.(check string) "the BVM Saturday office is observed"
+    "ef-time-after-pentecost-5-saturday" (S.to_string resolution.P.observed.P.cel.Cel.slug);
+  Alcotest.(check (list string)) "Mt Carmel is NOT commemorated (RG112(d))" []
+    (List.map (fun (c, _) -> S.to_string c.P.cel.Cel.slug) resolution.P.commemorations);
+  Alcotest.(check (list (pair string string))) "Mt Carmel is reported omitted, not silently dropped"
+    [ ("our-lady-of-mt-carmel", "omitted: yielded to a higher day") ]
+    (List.map (fun (c, reason) -> (S.to_string c.P.cel.Cel.slug, reason)) resolution.P.omitted)
+
 (* RG 16(a) (register §6.0) reached end-to-end through the REAL pipeline
    ([PE.band], [PE.disposition] AND [PE.admit] together, not any one of
    them in isolation as this file's own hand-tagged rows test them
@@ -1895,6 +2053,18 @@ let suite =
           "II-class Sunday override: RG109(b) witness admitted over an ordinary saint, end-to-end"
           `Quick test_ii_class_sunday_privileged_witness_admitted_end_to_end;
         Alcotest.test_case
+          "RG91 e27/RG78: the BVM Saturday office wins and commemorates Holy Machabees, end-to-end \
+           (2026-08-01)"
+          `Quick test_bvm_saturday_wins_and_commemorates_end_to_end;
+        Alcotest.test_case
+          "RG91 e27/RG78/RG26: the BVM Saturday office loses to a real feast and is omitted, end-to-end \
+           (2026-09-12)"
+          `Quick test_bvm_saturday_loses_to_a_real_feast_end_to_end;
+        Alcotest.test_case
+          "RG112(d), fix round 1 (F1): the BVM Saturday office excludes Mt Carmel, end-to-end \
+           (2033-07-16)"
+          `Quick test_bvm_saturday_excludes_mt_carmel_end_to_end;
+        Alcotest.test_case
           "RG16(a): a Feast of the Lord suppresses the Sunday's own commemoration entirely, \
            end-to-end (2028-08-06)"
           `Quick test_rg16a_lord_feast_suppresses_sunday_end_to_end;
diff --git a/test/test_temporal_ef.ml b/test/test_temporal_ef.ml
index 5394173..4e97d33 100644
--- a/test/test_temporal_ef.ml
+++ b/test/test_temporal_ef.ml
@@ -296,13 +296,31 @@ let test_holy_name_sunday_shape () =
                                      compared against)"
     None (Colitur_kernel.Names.find (office (d 2026 1 4)).Cel.names en);
   (* Negative controls: every OTHER day in the 2-5 January window that year
-     stays [subject Temporal] -- the override is scoped to exactly the one
-     Sunday [T.holy_name_sunday] names, not the whole window. *)
+     is NOT Lord -- RG 17(a)'s own override is scoped to exactly the one
+     Sunday [T.holy_name_sunday] names, not the whole window. 2 and 5 January
+     (Friday, Monday) stay plain [Temporal] ferias, as before. 3 January is a
+     SATURDAY -- an unrelated rubric (RG 78, "De sancta Maria in sabbato",
+     {!bvm_saturday_names}'s own citation in temporal_ef.ml) legitimately
+     retags it [Bvm], not [Temporal]: it is an otherwise-unoccupied IV-class
+     Saturday (Christmastide's "ef-christmas-1-*" ferial branch, [ferial_rank]'s
+     unconditional Class4 there), so RG 78 fires regardless of RG 17(a) being
+     in play the same week -- the two rubrics are independent and this is not
+     a collision. This is deliberately NOT folded into the [Temporal] check
+     above: asserting "stays Temporal" for 3 January would be WRONG once RG
+     78 is built, not merely incomplete. *)
   List.iter
     (fun dd ->
       Alcotest.(check bool) (Printf.sprintf "2026-01-%02d: NOT Lord (not the Sunday)" dd) true
-        (subject_of (d 2026 1 dd) = Sub.Temporal))
+        (subject_of (d 2026 1 dd) <> Sub.Lord))
     [ 2; 3; 5 ];
+  Alcotest.(check bool) "2026-01-02: plain Temporal feria (not a Saturday)" true
+    (subject_of (d 2026 1 2) = Sub.Temporal);
+  Alcotest.(check bool) "2026-01-05: plain Temporal feria (not a Saturday)" true
+    (subject_of (d 2026 1 5) = Sub.Temporal);
+  Alcotest.(check bool) "2026-01-03: a Saturday -- RG 78's BVM Saturday office fires here \
+                          independently of RG 17(a)"
+    true
+    (subject_of (d 2026 1 3) = Sub.Bvm);
   (* 2027: 3 January is the Sunday (`date -d 2027-01-03 +%u` = 7) -- the
      oracle window's own second instance of the Sunday shape. missalemeum's
      2027-01-03 row independently corroborates: rank 2, white, title "Holy
@@ -489,14 +507,43 @@ let test_colours () =
    (6 Jan) is a Tuesday, so the actual first-Sunday-after-Epiphany origin is
    11 Jan -- exercising all four sub-stretches of the fix. *)
 let test_christmastide_feria_slugs () =
+  (* 26 Dec 2026 is a SATURDAY -- but, fix round 1 (coordinator finding F2),
+     it can NEVER actually hold the BVM Saturday office in the real
+     resolved pipeline: 26/27/28 December are St Stephen/St John/the Holy
+     Innocents (RG 67-70), fixed, unconditional, real data/ef/sanctoral.sexp
+     entries every single year (rank >= Class2), which always outrank a
+     Class4 temporal candidate -- so RG 78's own protasis ("in quibus
+     OCCURRIT Officium de feria IV classis") is never satisfied by ANY day
+     in the 26-28 December stretch, in any year. [T.temporal] itself has no
+     visibility into the sanctoral layer at all (by design -- the temporal
+     candidate is computed independent of what saint's day it is; Precedence
+     decides the winner afterward), so it still unconditionally tags this
+     date [subject = Bvm] -- a genuine, correct fact about [T.temporal]'s
+     own pure computation, but NOT a claim that this shape is ever the real
+     OBSERVED office, and pinning it with RG-78 citation language as if it
+     were would overclaim. Only the SLUG is asserted here (unaffected
+     either way, unconditionally correct); the override demonstration
+     itself uses 3 January and 10 January below instead -- both, checked
+     directly against data/ef/sanctoral.sexp, have no competing entry in
+     any year, so the BVM Saturday office genuinely IS reachable there. *)
   Alcotest.(check string) "26 Dec (Sat)" "ef-christmas-0-saturday" (slug_of (d 2026 12 26));
   Alcotest.(check string) "28 Dec (Mon)" "ef-christmas-0-monday" (slug_of (d 2026 12 28));
+  (* 3 Jan 2026 is a SATURDAY, genuinely reachable (data/ef/sanctoral.sexp
+     has no 3 January entry) -- the real demonstration for the "-1-"
+     stretch that 26 December's own Saturday could not honestly provide. *)
+  Alcotest.(check string) "3 Jan (Sat): slug stays the ordinary \"-1-\" naming" "ef-christmas-1-saturday"
+    (slug_of (d 2026 1 3));
+  Alcotest.(check string) "3 Jan (Sat): the BVM Saturday office overrides the season colour (white, RG 120(b))"
+    "white" (colour_of (d 2026 1 3));
+  Alcotest.(check bool) "3 Jan (Sat): subject Bvm" true (subject_of (d 2026 1 3) = Sub.Bvm);
   Alcotest.(check string) "2 Jan (Fri)" "ef-christmas-1-friday" (slug_of (d 2026 1 2));
   Alcotest.(check string) "5 Jan (Mon)" "ef-christmas-1-monday" (slug_of (d 2026 1 5));
   (* 7-10 Jan precede the actual origin Sunday (11 Jan): colitur-only, not
-     lectio's "week 1" key, precisely to avoid colliding with 12-13 Jan below. *)
+     lectio's "week 1" key, precisely to avoid colliding with 12-13 Jan below.
+     10 January is ALSO genuinely reachable (no sanctoral entry that date). *)
   Alcotest.(check string) "7 Jan (Wed, before the origin)" "ef-christmas-2-wednesday" (slug_of (d 2026 1 7));
   Alcotest.(check string) "10 Jan (Sat, before the origin)" "ef-christmas-2-saturday" (slug_of (d 2026 1 10));
+  Alcotest.(check bool) "10 Jan (Sat): subject Bvm" true (subject_of (d 2026 1 10) = Sub.Bvm);
   (* 12-13 Jan are on/after the origin: genuinely week 1, lectio's key. *)
   Alcotest.(check string) "12 Jan (Mon, on/after the origin)" "ef-time-after-epiphany-1-monday"
     (slug_of (d 2026 1 12));
@@ -813,6 +860,98 @@ let test_sacred_triduum_identity () =
   Alcotest.(check bool) "1666-04-22 Holy Thursday: subject Lord" true
     (subject_of (d 1666 4 22) = Sub.Lord)
 
+(* RG 91 entry 27 / RG 78-79 (Caput IX, "De sancta Maria in sabbato") --
+   {!Rite_ef.Temporal_ef.bvm_saturday_names}'s own citation has the full
+   primary-source argument (both photographic scans and the electronic
+   transcription for RG 78/79 themselves; both photographic scans for RG
+   120(b)'s colour rule and the Missal's own "Missae de sancta Maria in
+   sabbato" heading). This test proves it holds across every season family
+   [ferial_rank] can give Class4 outside a privileged run -- Christmastide,
+   Septuagesima, Time after Epiphany, Time after Pentecost and ordinary
+   Paschaltide -- and that it does NOT fire where RG 78's own protasis is
+   false (Advent/Lent/Passiontide Saturdays, already II/III class; Ember
+   Saturdays, II/I class; a non-Saturday day in the very same week). Every
+   date below was independently checked against `colitur day <year>`'s own
+   real output (data/ef/sanctoral.sexp + adjustments.sexp) before being
+   pinned here, exactly like test_golden.ml's own discipline -- not merely
+   asserted against [T.temporal]'s own predicted shape a second time. *)
+let test_bvm_saturday () =
+  (* Septuagesima, clean (no competing sanctoral candidate that year):
+     19 February 2028, Septuagesima I Saturday. Colour is the clearest
+     possible witness here -- Septuagesima's own season colour is violet
+     (RG 128(b)), so a white result can only come from RG 120(b)'s override,
+     never from [season_colour] agreeing by coincidence the way
+     Christmastide/Paschaltide do below. *)
+  Alcotest.(check string) "2028-02-19 (Septuagesima I Saturday): slug stays the ordinary ferial key"
+    "ef-septuagesima-1-saturday" (slug_of (d 2028 2 19));
+  Alcotest.(check string) "2028-02-19: class-4, unconditional (RG 91's own IV-class table position)"
+    "class-4" (rank_of (d 2028 2 19));
+  Alcotest.(check string) "2028-02-19: white overrides Septuagesima's own violet (RG 120(b), not RG 128(b))"
+    "white" (colour_of (d 2028 2 19));
+  Alcotest.(check bool) "2028-02-19: subject Bvm" true (subject_of (d 2028 2 19) = Sub.Bvm);
+  Alcotest.(check (option string)) "2028-02-19: Latin name, both photographic scans and the transcription, \
+                                     word for word"
+    (Some "Officium sanctae Mariae in sabbato") (name_la_of (d 2028 2 19));
+  (* Time after Epiphany, clean: 16 February 2030. Season colour there is
+     green (RG 127), like Time after Pentecost below -- a second,
+     independent witness that the override is season-blind, not merely
+     "true for Septuagesima's violet and Time after Pentecost's green
+     happen to share a fix". *)
+  Alcotest.(check string) "2030-02-16 (Time after Epiphany V Saturday): slug stays the ordinary ferial key"
+    "ef-time-after-epiphany-5-saturday" (slug_of (d 2030 2 16));
+  Alcotest.(check string) "2030-02-16: white overrides Time after Epiphany's own green (RG 127)" "white"
+    (colour_of (d 2030 2 16));
+  Alcotest.(check bool) "2030-02-16: subject Bvm" true (subject_of (d 2030 2 16) = Sub.Bvm);
+  (* Time after Pentecost, clean: 27 June 2026. *)
+  Alcotest.(check string) "2026-06-27 (Time after Pentecost IV Saturday): slug stays the ordinary ferial key"
+    "ef-time-after-pentecost-4-saturday" (slug_of (d 2026 6 27));
+  Alcotest.(check string) "2026-06-27: white overrides Time after Pentecost's own green (RG 127)" "white"
+    (colour_of (d 2026 6 27));
+  Alcotest.(check bool) "2026-06-27: subject Bvm" true (subject_of (d 2026 6 27) = Sub.Bvm);
+  (* Ordinary Paschaltide, clean, outside the privileged Easter octave: 18
+     April 2026. Season colour there is ALREADY white (RG 119) -- colour
+     alone cannot distinguish the override firing from it simply not being
+     needed, so subject/name are the load-bearing assertions on this one,
+     not colour (the SAME reasoning [test_holy_family]'s own "Christmastide
+     already white" gap does not arise for -- this is a genuine, deliberate
+     exception recorded here, not an oversight). *)
+  Alcotest.(check string) "2026-04-18 (Paschaltide II Saturday): slug stays the ordinary ferial key"
+    "ef-easter-2-saturday" (slug_of (d 2026 4 18));
+  Alcotest.(check string) "2026-04-18: white (coincides with Paschaltide's own RG 119 white -- not a \
+                            distinguishing witness on its own)"
+    "white" (colour_of (d 2026 4 18));
+  Alcotest.(check bool) "2026-04-18: subject Bvm (the real witness this date proves, since colour alone \
+                          cannot)"
+    true (subject_of (d 2026 4 18) = Sub.Bvm);
+  Alcotest.(check (option string)) "2026-04-18: Latin name present too" (Some "Officium sanctae Mariae in sabbato")
+    (name_la_of (d 2026 4 18));
+  (* Negative controls: RG 78's own protasis ("in quibus occurrit Officium
+     de feria IV classis") is false for these -- each is already II or III
+     class, never reaching Class4 at all, so none of them is overridden. *)
+  Alcotest.(check string) "2026-03-14 (an ordinary Lent Saturday): stays III class, violet, Temporal"
+    "class-3" (rank_of (d 2026 3 14));
+  Alcotest.(check string) "2026-03-14: violet, NOT overridden (RG 78 never applies -- already III class)"
+    "violet" (colour_of (d 2026 3 14));
+  Alcotest.(check bool) "2026-03-14: subject stays Temporal" true (subject_of (d 2026 3 14) = Sub.Temporal);
+  Alcotest.(check string) "2026-12-12 (an ordinary Advent Saturday, before 17 Dec): stays III class"
+    "class-3" (rank_of (d 2026 12 12));
+  Alcotest.(check bool) "2026-12-12: subject stays Temporal (not overridden)" true
+    (subject_of (d 2026 12 12) = Sub.Temporal);
+  Alcotest.(check string) "2026-02-28 (Lent Ember Saturday): stays II class, NOT overridden" "class-2"
+    (rank_of (d 2026 2 28));
+  Alcotest.(check bool) "2026-02-28: subject stays Temporal" true (subject_of (d 2026 2 28) = Sub.Temporal);
+  Alcotest.(check string) "2026-09-26 (September Ember Saturday): stays II class, NOT overridden" "class-2"
+    (rank_of (d 2026 9 26));
+  (* A non-Saturday day in the SAME week as a clean BVM Saturday stays an
+     ordinary ferial -- the override is exactly one weekday wide, not the
+     whole week. 26 June 2026 is the Friday immediately before the 27 June
+     Saturday pinned above. *)
+  Alcotest.(check string) "2026-06-26 (Friday, day before a BVM Saturday): plain ferial, not overridden"
+    "ef-time-after-pentecost-4-friday" (slug_of (d 2026 6 26));
+  Alcotest.(check string) "2026-06-26: green, the season's own colour, unaffected" "green"
+    (colour_of (d 2026 6 26));
+  Alcotest.(check bool) "2026-06-26: subject stays Temporal" true (subject_of (d 2026 6 26) = Sub.Temporal)
+
 let test_totality () =
   (* Every day of 2026 yields an office without raising. Not a slug
      re-validation -- Slug.t is a private string validated on construction,
@@ -848,6 +987,7 @@ let suite_extra =
     Alcotest.test_case "colours" `Quick test_colours;
     Alcotest.test_case "sacred triduum identity (RG 91 entry 2, Gap 1)" `Quick
       test_sacred_triduum_identity;
+    Alcotest.test_case "BVM Saturday office (RG 91 entry 27, RG 78-79)" `Quick test_bvm_saturday;
     Alcotest.test_case "christmastide feria slugs" `Quick test_christmastide_feria_slugs;
     Alcotest.test_case "named days carry their week" `Quick test_named_days_carry_their_week;
     Alcotest.test_case "totality" `Quick test_totality;