1 files changed, 57 insertions, 0 deletions
diff --git a/internal/calendar/temporal_ef_test.go b/internal/calendar/temporal_ef_test.go
index db6e6aa..7dd7400 100644
--- a/internal/calendar/temporal_ef_test.go
+++ b/internal/calendar/temporal_ef_test.go
@@ -147,3 +147,60 @@ func TestTemporalEFEmberDayRanks(t *testing.T) {
 		}
 	}
 }
+
+// TestTemporalEFChristmastideBoundary pins RG 72's boundary and, just as
+// importantly, the SEPARATION it required.
+//
+// RG 72: "Tempus natalicium decurrit a I Vesperis Nativitatis Domini usque ad
+// diem 13 ianuarii INCLUSIVE." Christmas Time runs to 13 January, so 6-13
+// January is the Epiphany section OF Christmas Time, not the beginning of Time
+// after Epiphany. RG 119(a) backs it from the colour side, white "usque ad
+// expletum tempus Epiphaniae".
+//
+// lectio ended Christmas Time on 5 January, so those eight days came out
+// green. The fix could not be the boundary constant alone: efSeason both named
+// the slugs and reported the season, so moving it renamed every slug in the
+// window and orphaned the ef-time-after-epiphany-sunday-1 lectionary key. The
+// season and the slug are now separate axes -- efSeason and efSlugSeason --
+// and this test pins BOTH, because a future "simplification" that merges them
+// again would pass any test that checked only one.
+func TestTemporalEFChristmastideBoundary(t *testing.T) {
+	for _, c := range []struct {
+		date, season, slug string
+	}{
+		// Epiphany itself is inside Christmas Time, not the start of what
+		// follows it. Its slug is unchanged.
+		{"2026-01-06", "christmas", "ef-epiphany"},
+		// The window: season christmas, slugs and week numbers still Epiphany's.
+		{"2026-01-07", "christmas", "ef-time-after-epiphany-1-wednesday"},
+		{"2026-01-11", "christmas", "ef-time-after-epiphany-sunday-1"},
+		{"2026-01-12", "christmas", "ef-time-after-epiphany-1-monday"},
+		// 13 January is INCLUSIVE -- the last day of Christmas Time. It carries
+		// the Commemoration of the Baptism, so only the season is asserted.
+		{"2026-01-13", "christmas", ""},
+		// 14 January is the first day that is genuinely Time after Epiphany.
+		{"2026-01-14", "time-after-epiphany", ""},
+		// The other end, unchanged: 5 January is Christmas Time by the old
+		// boundary too, so a regression that moved the boundary the wrong way
+		// would not show here -- which is why 14 January above is asserted.
+		{"2026-01-05", "christmas", ""},
+	} {
+		got := temporalEF(d(c.date))
+		if string(got.Season) != c.season {
+			t.Errorf("%s: season = %q, want %q (RG 72: Christmas Time runs to 13 January inclusive)",
+				c.date, got.Season, c.season)
+		}
+		if c.slug != "" && got.Cel.Slug != c.slug {
+			t.Errorf("%s: slug = %q, want %q -- the SLUG season must stay Epiphany's here; "+
+				"if this fails with an ef-christmas-* slug, efSeason and efSlugSeason have been "+
+				"merged again and the lectionary keys are orphaned", c.date, got.Cel.Slug, c.slug)
+		}
+	}
+	// The colour follows from the season, and is the reason any of this
+	// matters: eight days a year were green that should be white.
+	for _, day := range []string{"2026-01-07", "2026-01-09", "2026-01-12"} {
+		if got := temporalEF(d(day)).Cel.Colour; got != White {
+			t.Errorf("%s: colour = %s, want white (RG 119(a), following from the season)", day, got)
+		}
+	}
+}