test(calendar): regression oracle vs calapi 2020-2040 (0 season mismatches)

Caught + fixed an Advent-start bug: anchor on Christmas Eve, not Dec 25, so
years where Dec 25 is a Sunday don't lose the first week of Advent. Rank-class
diffs (324) are informational — the initial sanctoral dataset is partial.

1 files changed, 8 insertions, 4 deletions

diff --git a/internal/calendar/temporal.go b/internal/calendar/temporal.go
index 568efdc..b14e974 100644
--- a/internal/calendar/temporal.go
+++ b/internal/calendar/temporal.go
@@ -23,11 +23,15 @@ type temporalDay struct {
 }
 
 // adventStart returns the First Sunday of Advent that opens the liturgical year
-// containing Christmas of the given calendar year: the 4th Sunday before Dec 25.
+// containing Christmas of the given calendar year. The 4th Sunday of Advent is
+// the Sunday on or before Christmas Eve (Dec 24) — anchoring on Dec 24, not
+// Dec 25, keeps Christmas Day itself from being counted as an Advent Sunday
+// (which loses a week in years when Dec 25 is a Sunday). Advent I is 3 Sundays
+// earlier.
 func adventStart(year int) time.Time {
-	xmas := time.Date(year, 12, 25, 0, 0, 0, 0, time.UTC)
-	sun := xmas.AddDate(0, 0, -int(xmas.Weekday())) // Sunday on/before Christmas (Sunday=0)
-	return sun.AddDate(0, 0, -21)
+	eve := time.Date(year, 12, 24, 0, 0, 0, 0, time.UTC)
+	sun := eve.AddDate(0, 0, -int(eve.Weekday())) // 4th Sunday of Advent
+	return sun.AddDate(0, 0, -21)                  // 1st Sunday of Advent
 }
 
 func sameDay(a, b time.Time) bool {