fix(ef): the weekdays within the Octave of the Nativity have their own Mass

29, 30 and 31 December are named days in the Missal's calendarium -- "De V die
infra octavam Nativitatis Domini, II classis" and so on -- with a direct
formulary, "Diebus infra octavam Nativitatis Domini": Titus 3:4-7 /
Luke 2:15-20, and each date's own rubric pointing at it ("Die 29 decembris ...
Missa Puer natus est nobis, ut supra").

lectio treated them as ordinary Christmas ferias, so they fell back to the
preceding Sunday and served "Dominica infra octavam Nativitatis Domini"
(Gal 4:1-7 / Luke 2:33-40) -- a different heading, for the Sunday
specifically.

Worth recording because it is not a guess: colitur made the IDENTICAL mistake,
gave these three days the Sunday's citation, measured it against real years
and reverted it. Its bootstrap_lectionary.ml keeps the rejection on record
with the scan references for both headings, which is how this one was
diagnosed rather than re-derived.

Only the weekdays. When 29-31 December falls on a Sunday the day really IS
Dominica infra octavam and keeps the Sunday's Mass -- 2030-12-29 is the case,
and it is asserted, because a fix applied by date alone would pass on the
weekdays and silently swallow the Sunday. Mutation-tested: dropping the Sunday
exclusion reddens exactly that assertion and nothing else.

1 files changed, 45 insertions, 0 deletions

diff --git a/internal/caldata/caldata_test.go b/internal/caldata/caldata_test.go
index f993a12..2a13d3d 100644
--- a/internal/caldata/caldata_test.go
+++ b/internal/caldata/caldata_test.go
@@ -521,3 +521,48 @@ func TestEFBVMSaturdayReadings(t *testing.T) {
 		t.Errorf("2026-04-18: gospel = %q, want %q (the Paschaltide formulary)", gospel, "John 19:25-27")
 	}
 }
+
+// TestEFNativityOctaveWeekdayMass pins the Mass of a WEEKDAY within the Octave
+// of the Nativity, and the Sunday it must not swallow.
+//
+// The Missal gives 29-31 December a direct formulary, "Diebus infra octavam
+// Nativitatis Domini, II classis" -- Titus 3:4-7 / Luke 2:15-20 -- and names
+// them in the calendarium ("De V die infra octavam ... II classis"). Treated
+// as ordinary ferias they fell back to the preceding Sunday and served
+// "Dominica infra octavam" (Gal 4:1-7 / Luke 2:33-40), which is a different
+// heading for a different day.
+//
+// The Sunday case is asserted too, because it is the one the wrong citation
+// blurred: when 29-31 December IS a Sunday the day really is "Dominica infra
+// octavam" and keeps that Mass. A fix that applied the weekday formulary by
+// date alone would pass the first half of this test and fail the second.
+func TestEFNativityOctaveWeekdayMass(t *testing.T) {
+	sel := calendar.Selection{Form: "old"}
+	layers := []calendar.Layer{Tridentine()}
+	get := func(date string) (string, string) {
+		day, _ := time.Parse("2006-01-02", date)
+		var first, gospel string
+		for _, r := range Readings(sel, layers, day, calendar.Compute(day, sel, layers)) {
+			switch r.Part {
+			case "first":
+				first = r.Citation
+			case "gospel":
+				gospel = r.Citation
+			}
+		}
+		return first, gospel
+	}
+	// 2026: 29-31 December are Tuesday, Wednesday, Thursday -- weekdays.
+	for _, date := range []string{"2026-12-29", "2026-12-30", "2026-12-31"} {
+		if f, g := get(date); f != "Titus 3:4-7" || g != "Luke 2:15-20" {
+			t.Errorf("%s: got %q / %q, want Titus 3:4-7 / Luke 2:15-20 "+
+				"(Diebus infra octavam Nativitatis Domini)", date, f, g)
+		}
+	}
+	// 2030: 29 December is a SUNDAY, so it is Dominica infra octavam and keeps
+	// the Sunday's own Mass.
+	if f, g := get("2030-12-29"); f != "Gal 4:1-7" || g != "Luke 2:33-40" {
+		t.Errorf("2030-12-29 (a Sunday): got %q / %q, want Gal 4:1-7 / Luke 2:33-40 "+
+			"-- the weekday formulary must not swallow Dominica infra octavam", f, g)
+	}
+}