1 files changed, 53 insertions, 1 deletions

diff --git a/internal/calendar/temporal_ef_test.go b/internal/calendar/temporal_ef_test.go
index 7dd7400..a7fa8b0 100644
--- a/internal/calendar/temporal_ef_test.go
+++ b/internal/calendar/temporal_ef_test.go
@@ -1,6 +1,9 @@
 package calendar
 
-import "testing"
+import (
+	"testing"
+	"time"
+)
 
 func TestTemporalEFSeasons(t *testing.T) {
 	cases := []struct {
@@ -204,3 +207,52 @@ func TestTemporalEFChristmastideBoundary(t *testing.T) {
 		}
 	}
 }
+
+// TestTemporalEFEpiphanyWeekAnchor pins where the weeks after Epiphany are
+// counted from. They are numbered from the FIRST SUNDAY after Epiphany, not
+// from Epiphany itself -- the Sunday carries the Mass the week is named for
+// ("Dominica I post Epiphaniam"), while Epiphany is a feast inside Christmas
+// Time (RG 72), not the head of a numbered week.
+//
+// Counting from 6 January put every week one too high, all year until
+// Septuagesima cut the season short.
+func TestTemporalEFEpiphanyWeekAnchor(t *testing.T) {
+	// 2026: Epiphany is a Tuesday, so the first Sunday after it is 11 January.
+	for _, c := range []struct {
+		date string
+		week int
+	}{
+		{"2026-01-14", 1}, // Wednesday of the week running from 11 January
+		{"2026-01-17", 1}, // still that week
+		{"2026-01-20", 2},
+		{"2026-01-27", 3},
+	} {
+		if got := temporalEF(d(c.date)).Week; got != c.week {
+			t.Errorf("%s: week = %d, want %d (weeks after Epiphany count from the "+
+				"first Sunday after it, not from 6 January)", c.date, got, c.week)
+		}
+	}
+	// 2028: Epiphany falls ON a Thursday and 9 January is the Sunday; but the
+	// year that matters for the off-by-one is one where Epiphany is itself a
+	// Sunday, since then "the first Sunday AFTER Epiphany" is the following
+	// week and a naive same-day anchor would be seven days out.
+	y := 0
+	for c := 2005; c <= 2050; c++ {
+		if time.Date(c, time.January, 6, 0, 0, 0, 0, time.UTC).Weekday() == time.Sunday {
+			y = c
+			break
+		}
+	}
+	if y == 0 {
+		t.Fatal("no year in 2005..2050 has Epiphany on a Sunday -- adjust this test")
+	}
+	anchor := firstSundayAfterEpiphany(y)
+	if anchor.Day() == 6 {
+		t.Errorf("%d: Epiphany is a Sunday and the anchor landed on it (%s); the "+
+			"first Sunday AFTER Epiphany is the next one", y, anchor.Format("2006-01-02"))
+	}
+	if got := temporalEF(anchor).Week; got != 1 {
+		t.Errorf("%d: the first Sunday after Epiphany (%s) should be week 1, got %d",
+			y, anchor.Format("2006-01-02"), got)
+	}
+}