fix(of): Nativity Vigil no longer suppresses a Sunday of Advent

Temporal_of.named claimed 24 December unconditionally as
of-nativity-vigil (Sollemnitas), and temporal's dispatch matched named
before sunday_slug. On a Sunday 24 December, the Fourth Sunday of
Advent -- Tabula I.2, Normae n. 5's own privileged Sundays of Advent,
Lent and Easter -- was silently replaced by the Vigil.

named now returns None for 24 December when that date is itself a
Sunday, so temporal's dispatch falls through to sunday_slug and the
day resolves as of-advent-sunday-4. The Vigil Mass is an evening Mass
of the following solemnity, not an office belonging to 24 December's
own civil day -- the same civil-day-granularity reasoning already
applied to Holy Thursday (Precedence_of.band's entry-1 comment) and to
expected-divergences-litcal.sexp's L1 entry. Every non-Sunday
24 December is unaffected.

Audited every other named entry for the same latent shape (see
.superpowers/sdd/of-r8-advent-iv-report.md): none of them has it,
either because the date can never fall on a Sunday, or because the
date IS the definition of that Sunday (Palm Sunday, Easter, Pentecost,
Trinity, Christ the King), or because the competing movable-Sunday
windows are constructed to exclude that date.

Added test_nativity_vigil_yields_to_advent_sunday, covering
2028-12-24 and 2034-12-24 (both inside the litcal fixture window) and
the 2026-12-24 non-Sunday control. Re-ran the litcal layer: no
allow-list counts moved, since it compares only season and
Ordinary-Time week, and both readings of 24 December are "advent"
either way. Updated test_precedence_of.ml's "entry 3, not 9" comment,
which had carried this as a known, unfixed defect, to describe the
fix; its own pinned assertion (a hand-built Thursday candidate) needed
no change.

4 files changed, 85 insertions, 13 deletions

diff --git a/lib/rites/rite_of/temporal_of.ml b/lib/rites/rite_of/temporal_of.ml
index 9aa898e..e375350 100644
--- a/lib/rites/rite_of/temporal_of.ml
+++ b/lib/rites/rite_of/temporal_of.ml
@@ -238,7 +238,33 @@ let named d =
   let easter = Computus.gregorian_easter y in
   let off n = Date.add_days easter n in
   let m = Date.month d and dd = Date.day d in
-  if m = 12 && dd = 24 then Some (Advent, "of-nativity-vigil", Colour.White, Sollemnitas)
+  if m = 12 && dd = 24 && Date.weekday d <> Date.Sun then
+    Some (Advent, "of-nativity-vigil", Colour.White, Sollemnitas)
+    (* NOT unconditional. Tabula I.2 ("Dominicae Adventus, Quadragesimae et
+       Paschae") outranks Tabula I.3 (solemnities generally, where the Vigil
+       Mass of the Nativity itself bands, per Precedence_of.band's own entry-3
+       branch) -- Normae n. 5 states it directly for exactly this class of
+       day: a Sunday of Advent/Lent/Easter yields to nothing but a still
+       higher-banked day. The Fourth Sunday of Advent is one of those
+       privileged Sundays every time 24 December falls on one.
+
+       The Vigil Mass is an evening Mass of the FOLLOWING solemnity (the
+       Nativity, 25 December), not an office belonging to 24 December's own
+       civil day -- the identical granularity reasoning
+       Precedence_of.band's own entry-1 comment already applies to Holy
+       Thursday (the Triduum opens with an evening Mass inside Holy
+       Thursday's civil day, so the day itself stays at entry 2, not entry
+       1) and that data/of/expected-divergences-litcal.sexp's L1 entry
+       applies on the season axis. At colitur's one-office-per-civil-day
+       granularity, only one identity can hold 24 December, and on a Sunday
+       that identity is the Fourth Sunday of Advent, not the Vigil -- so
+       [named] must return [None] here and let [temporal]'s dispatch fall
+       through to [sunday_slug], exactly as it already does for every other
+       Sunday [named] does not claim.
+
+       Every non-Sunday 24 December is unaffected: the Vigil still wins
+       there, as a plain late-Advent feria (Tabula II.9) is never privileged
+       against it. *)
   else if m = 12 && dd = 25 then Some (Christmas, "of-nativity", Colour.White, Sollemnitas)
   else if m = 1 && dd = 1 then Some (Christmas, "of-mary-mother-of-god", Colour.White, Sollemnitas)
   else if m = 1 && dd = 6 then Some (Christmas, "of-epiphany", Colour.White, Sollemnitas)
diff --git a/lib/rites/rite_of/temporal_of.mli b/lib/rites/rite_of/temporal_of.mli
index 982ff05..d2acadd 100644
--- a/lib/rites/rite_of/temporal_of.mli
+++ b/lib/rites/rite_of/temporal_of.mli
@@ -103,7 +103,16 @@ val christ_the_king : int -> Date.t
     threading extra state through every other branch too -- the exact
     reason EF's own [named] excludes its structurally identical Holy
     Family/Holy Name. {!temporal} applies all three as its own targeted
-    dispatch instead. *)
+    dispatch instead.
+
+    24 December is ALSO conditional, not unconditional like the rest of this
+    function's fixed dates: it yields [of-nativity-vigil] only when that date
+    is not itself a Sunday. On a Sunday, [None] is returned so {!temporal}'s
+    dispatch falls through to {!sunday_slug}'s own [of-advent-sunday-4] --
+    Tabula I.2/Normae n. 5's Sundays of Advent outrank the Vigil (Tabula
+    I.3), the same evening-Mass-inside-the-civil-day reasoning already
+    applied to Holy Thursday (see [named]'s own implementation comment for
+    the full argument). *)
 val named : Date.t -> (Vocab_of.season * string * Colour.t * Vocab_of.rank) option
 
 (** Week within the season, 1-indexed, Sunday-aligned (constant across a
diff --git a/test/test_precedence_of.ml b/test/test_precedence_of.ml
index 4de9f9a..610cbfb 100644
--- a/test/test_precedence_of.ml
+++ b/test/test_precedence_of.ml
@@ -162,17 +162,31 @@ let part2_cases =
        entry 3. The pinned assertion above is still correct for the Thursday
        it names; only the earlier "band 30" framing read as unconditional,
        which it is not.
-       There is a real, KNOWN, UNFIXED Phase 1 defect behind this: [named]
-       (temporal_of.ml) claims 24 December unconditionally and is matched
-       BEFORE [sunday_slug] in [temporal]'s dispatch, so on a 24-December
-       Sunday the Fourth Sunday of Advent gets NO office at all -- it is
-       silently replaced by [of-nativity-vigil]. 2028-12-24 and 2034-12-24
-       both fall inside the litcal fixture window, and that layer cannot see
-       the gap: it compares season and Ordinary-Time week only, and colitur
-       still says "advent" on that date either way. NOT fixed in this wave
-       (final review Important #4, ledger ruling R8) -- it changes Phase 1
-       behaviour, needs its own tests, and needs the litcal layer re-run.
-       Carried as the first item of the next task. *)
+       RESOLVED (task R8, 2026-08-25): the Phase 1 defect this paragraph used
+       to describe -- [named] (temporal_of.ml) claiming 24 December
+       unconditionally, matched BEFORE [sunday_slug] in [temporal]'s
+       dispatch, so a 24-December Sunday silently lost the Fourth Sunday of
+       Advent to [of-nativity-vigil] -- is fixed. [named] now returns [None]
+       for 24 December when that date is itself a Sunday, citing Tabula
+       I.2/Normae n. 5 and the identical Holy Thursday granularity argument
+       (b) above already makes; [temporal]'s dispatch then falls through to
+       [sunday_slug] exactly as it does for every other Sunday [named] does
+       not claim, landing on [of-advent-sunday-4]. Paragraph (d) above
+       already anticipated the RIGHT band (20, via [is_privileged_sunday])
+       for that case; this fix is what makes that band actually reachable
+       from a real [temporal] call rather than only from this row's own
+       hand-built candidate. Confirmed on both 2028-12-24 and 2034-12-24
+       (test_temporal_of.ml, "nativity vigil yields to Advent Sunday IV"):
+       [named] claims nothing, [temporal] resolves to [of-advent-sunday-4],
+       and the non-Sunday case (2026-12-24 here) is unchanged. The litcal
+       layer was re-run: no allow-list count moved (L1's 36, the 1876
+       witnessed Ordinary Time weeks, and the 859 unwitnessed count are all
+       unchanged), because that layer compares season and Ordinary-Time week
+       only and both readings of 24 December are "advent" either way -- the
+       exact blind spot this same paragraph named above. This row's own
+       pinned assertion (a hand-built candidate, Thursday 2026-12-24, band
+       30) needed no change: it was never wrong, only the claim that no test
+       anywhere exercised the Sunday case. *)
     ( "entry 3, not 9: the Nativity Vigil is a solemnity, though 24 December \
        is also a late-Advent feria",
       ctx ~iso:"2026-12-24" ~season:Vocab_of.Advent,
diff --git a/test/test_temporal_of.ml b/test/test_temporal_of.ml
index 7560413..1ccd098 100644
--- a/test/test_temporal_of.ml
+++ b/test/test_temporal_of.ml
@@ -230,6 +230,28 @@ let test_ferial_slugs () =
   Alcotest.(check string) "ordinary weekday" "of-ordinary-time-2-monday" (slug_of (d 2026 1 19));
   Alcotest.(check string) "lent weekday" "of-lent-6-monday" (slug_of (d 2026 3 30))
 
+(* R8 fix: a Sunday 24 December must yield the Fourth Sunday of Advent, not
+   the Vigil -- Tabula I.2/Normae n. 5, and temporal_of.ml's own [named]
+   comment for the full argument (the same civil-day-granularity reasoning
+   already applied to Holy Thursday). 2028-12-24 and 2034-12-24 are the two
+   such Sundays inside the litcal fixture window (2023-12-03..2035-12-01);
+   2026-12-24, a Thursday, is the non-Sunday control. *)
+let test_nativity_vigil_yields_to_advent_sunday () =
+  List.iter
+    (fun (y, label) ->
+      let dt = d y 12 24 in
+      Alcotest.(check bool) (label ^ ": 24 Dec is a Sunday") true (D.weekday dt = D.Sun);
+      Alcotest.(check string) (label ^ ": named claims nothing") "<none>" (named_slug dt);
+      Alcotest.(check string) (label ^ ": temporal resolves to Advent IV") "of-advent-sunday-4" (slug_of dt);
+      Alcotest.(check string) (label ^ ": season is still advent") "advent" (season_str dt);
+      Alcotest.(check (option int)) (label ^ ": week 4") (Some 4) (T.week dt))
+    [ (2028, "2028"); (2034, "2034") ];
+  (* Non-Sunday 24 December: unaffected, the Vigil still wins. *)
+  let thu = d 2026 12 24 in
+  Alcotest.(check bool) "2026: 24 Dec is a Thursday, not a Sunday" true (D.weekday thu <> D.Sun);
+  Alcotest.(check string) "2026: named still claims the vigil" "of-nativity-vigil" (named_slug thu);
+  Alcotest.(check string) "2026: temporal still resolves to the vigil" "of-nativity-vigil" (slug_of thu)
+
 let test_colours () =
   let colour_of dt = (T.temporal dt).Colitur_kernel.Temporal.office.Colitur_kernel.Celebration.colour in
   Alcotest.(check string) "Good Friday is red" "red" (Colitur_kernel.Colour.to_string (colour_of (d 2026 4 3)));
@@ -387,6 +409,7 @@ let suite =
       Alcotest.test_case "ordinary time resumption" `Quick test_ordinary_time_resumption;
       Alcotest.test_case "sunday slugs" `Quick test_sunday_slugs;
       Alcotest.test_case "ferial slugs" `Quick test_ferial_slugs;
+      Alcotest.test_case "nativity vigil yields to Advent Sunday IV" `Quick test_nativity_vigil_yields_to_advent_sunday;
       Alcotest.test_case "colours" `Quick test_colours;
       Alcotest.test_case "domain edges do not raise" `Quick test_domain_edges_do_not_raise;
       Alcotest.test_case "exhaustive domain sweep" `Slow test_exhaustive_domain_sweep ]