2 files changed, 76 insertions, 0 deletions
diff --git a/internal/calendar/temporal_ef.go b/internal/calendar/temporal_ef.go
index 6828f35..0cd4b41 100644
--- a/internal/calendar/temporal_ef.go
+++ b/internal/calendar/temporal_ef.go
@@ -159,6 +159,23 @@ func temporalEF(date time.Time) temporalDay {
 		return efCel(Christmas, "ef-nativity", White, RankClass1, 0)
 	case date.Month() == time.January && date.Day() == 1:
 		return efCel(Christmas, "ef-circumcision", White, RankClass1, 0)
+	// RG 17(a): "festum Ss.mi Nominis Iesu, celebrandum dominica quae occurrit
+	// a die 2 ad 5 ianuarii, secus die 2 ianuarii" -- the Most Holy Name of
+	// Jesus is kept on the Sunday falling 2-5 January, and when no Sunday falls
+	// in that window, on 2 January instead.
+	//
+	// The fallback is not decoration: the window is four days wide, so in
+	// roughly three years in seven no Sunday lands in it and 2 January carries
+	// the feast. Without the fallback lectio simply had no Holy Name office at
+	// all in those years.
+	//
+	// Placed before the Epiphany case below and after the Circumcision above,
+	// so 1 January keeps its own feast and 6 January keeps Epiphany.
+	case date.Month() == time.January && date.Day() >= 2 && date.Day() <= 5 &&
+		date.Weekday() == time.Sunday:
+		return efCel(Christmas, "ef-holy-name-sunday", White, RankClass2, 0)
+	case date.Month() == time.January && date.Day() == 2 && !sundayFallsJan2to5(y):
+		return efCel(Christmas, "ef-holy-name", White, RankClass2, 0)
 	case date.Month() == time.January && date.Day() == 6:
 		// Christmas, not TimeAfterEpiphany: RG 72 puts Epiphany INSIDE Christmas
 		// Time, which runs to 13 January. The slug is unchanged.
@@ -330,6 +347,19 @@ func temporalEF(date time.Time) temporalDay {
 	return efCel(season, "ef-"+string(slugSeason)+"-"+strconv.Itoa(week)+"-"+weekdayLower(date), col, rank, week)
 }
 
+// sundayFallsJan2to5 reports whether any Sunday falls in RG 17(a)'s own window,
+// 2 to 5 January inclusive. When one does the Holy Name is kept on it; when
+// none does the feast falls back to 2 January, which is what "secus die 2
+// ianuarii" provides for.
+func sundayFallsJan2to5(y int) bool {
+	for day := 2; day <= 5; day++ {
+		if time.Date(y, time.January, day, 0, 0, 0, 0, time.UTC).Weekday() == time.Sunday {
+			return true
+		}
+	}
+	return false
+}
+
 // firstSundayAfterEpiphany returns the first Sunday strictly after 6 January
 // of year y -- the anchor the weeks after Epiphany are numbered from. When
 // Epiphany itself falls on a Sunday the next one is meant, which is why the
diff --git a/internal/calendar/temporal_ef_test.go b/internal/calendar/temporal_ef_test.go
index d930cc7..c1571b0 100644
--- a/internal/calendar/temporal_ef_test.go
+++ b/internal/calendar/temporal_ef_test.go
@@ -360,3 +360,49 @@ func TestTemporalEFPassiontideWeeks(t *testing.T) {
 		}
 	}
 }
+
+// TestTemporalEFHolyName pins RG 17(a): "festum Ss.mi Nominis Iesu,
+// celebrandum dominica quae occurrit a die 2 ad 5 ianuarii, secus die 2
+// ianuarii" -- the Most Holy Name of Jesus is kept on the Sunday falling 2-5
+// January, and when no Sunday falls in that window, on 2 January instead.
+//
+// The FALLBACK is the half worth testing hardest. The window is four days
+// wide, so in about three years in seven no Sunday lands in it; without the
+// fallback there is simply no Holy Name office in those years, which is a
+// missing II-class feast rather than a misnamed one.
+func TestTemporalEFHolyName(t *testing.T) {
+	// Years where a Sunday does fall in the window, at each end of it.
+	for _, c := range []struct{ date, slug string }{
+		{"2026-01-04", "ef-holy-name-sunday"}, // Sunday the 4th
+		{"2027-01-03", "ef-holy-name-sunday"}, // Sunday the 3rd
+		{"2028-01-02", "ef-holy-name-sunday"}, // Sunday the 2nd -- the window's first day
+	} {
+		if got := temporalEF(d(c.date)).Cel.Slug; got != c.slug {
+			t.Errorf("%s: slug = %q, want %q", c.date, got, c.slug)
+		}
+	}
+	// 2029: no Sunday falls 2-5 January, so 2 January carries the feast.
+	if got := temporalEF(d("2029-01-02")).Cel.Slug; got != "ef-holy-name" {
+		t.Errorf("2029-01-02: slug = %q, want ef-holy-name -- no Sunday falls 2-5 January "+
+			"that year, so RG 17(a)'s \"secus die 2 ianuarii\" applies", got)
+	}
+	// And in a year where a Sunday DOES fall in the window, 2 January must NOT
+	// also carry it -- the fallback is an alternative, not an addition.
+	if got := temporalEF(d("2026-01-02")).Cel.Slug; got == "ef-holy-name" {
+		t.Errorf("2026-01-02: slug = %q, but the Sunday the 4th carries the feast that "+
+			"year -- the fallback must not fire as well", got)
+	}
+	// Sanity on the helper itself, across the whole fixture range: every year
+	// gets the feast exactly once, by one shape or the other.
+	for y := 2005; y <= 2050; y++ {
+		n := 0
+		for day := 2; day <= 5; day++ {
+			if s := temporalEF(time.Date(y, time.January, day, 0, 0, 0, 0, time.UTC)).Cel.Slug; s == "ef-holy-name" || s == "ef-holy-name-sunday" {
+				n++
+			}
+		}
+		if n != 1 {
+			t.Errorf("%d: the Holy Name appears %d times in 2-5 January, want exactly 1", y, n)
+		}
+	}
+}