2 files changed, 65 insertions, 3 deletions
diff --git a/internal/calendar/calendar.go b/internal/calendar/calendar.go
index 98dec43..e0f5217 100644
--- a/internal/calendar/calendar.go
+++ b/internal/calendar/calendar.go
@@ -6,11 +6,49 @@ import (
 	"time"
 )
 
-// Compute returns the Ordinary Form liturgical day for date under the given
-// Selection, with the ordered layer stack supplying celebrations. It is pure
-// and total: any valid Gregorian date yields a LiturgicalDay.
+// Compute returns the liturgical day for date under the given Selection, with
+// the ordered layer stack supplying celebrations. It is pure and total: any
+// valid Gregorian date yields a LiturgicalDay. Selection.Form "old" computes the
+// Extraordinary Form (1962); anything else computes the Ordinary Form.
 func Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
 	date = date.UTC().Truncate(24 * time.Hour)
+	if sel.Form == "old" {
+		return computeEF(date, sel, layers)
+	}
+	return computeOF(date, sel, layers)
+}
+
+// computeEF computes the Extraordinary Form (1962) day.
+func computeEF(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
+	td := temporalEF(date)
+	merged := mergeLayers(layers)
+	cands := []candidate{{Cel: td.Cel, Temporal: true, Season: td.Season}}
+	for slug, rc := range merged {
+		cel := buildCelebration(slug, rc)
+		when, ok := celebrationDate(cel, date.Year(), sel)
+		if !ok {
+			continue
+		}
+		if sameDay(when, date) {
+			cands = append(cands, candidate{Cel: cel, Temporal: false, Season: td.Season})
+		}
+	}
+	obs, others := pickEF(cands)
+	day := LiturgicalDay{
+		Date: date, Season: td.Season, Week: td.Week, Weekday: date.Weekday(),
+		Observed: obs.Cel, Colour: colourOf(obs, td), ObservedBand: precedenceEF(obs),
+	}
+	for _, o := range others {
+		if o.Temporal {
+			continue
+		}
+		day.Others = append(day.Others, o.Cel)
+	}
+	return day
+}
+
+// computeOF computes the Ordinary Form day.
+func computeOF(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
 	td := temporal(date, sel)
 	merged := mergeLayers(layers)
 
diff --git a/internal/calendar/calendar_test.go b/internal/calendar/calendar_test.go
index 909f1d8..28dba9e 100644
--- a/internal/calendar/calendar_test.go
+++ b/internal/calendar/calendar_test.go
@@ -37,3 +37,27 @@ func TestComputeCycles(t *testing.T) {
 		t.Errorf("2025 weekday cycle = %s want I", got.WeekdayCycle)
 	}
 }
+
+func TestComputeEF(t *testing.T) {
+	sel := DefaultSelection()
+	sel.Form = "old"
+	// an EF layer with a I-class feast on 2025-08-15 (Assumption)
+	layer := Layer{ID: "tridentine", Type: "universal", Cels: map[string]RawCelebration{
+		"assumption": {Fields: map[string]string{
+			"date": "08-15", "rank": "class-1", "colour": "white", "name.en": "Assumption BVM",
+		}, Variants: map[string]map[string]string{}},
+	}}
+	got := Compute(d("2025-08-15"), sel, []Layer{layer})
+	if got.Observed.Slug != "assumption" || got.Observed.Rank != RankClass1 {
+		t.Fatalf("EF 2025-08-15 observed = %q/%v want assumption/class-1", got.Observed.Slug, got.Observed.Rank)
+	}
+	// the EF season on 2025-08-15 is Time after Pentecost (not OF's "ordinary")
+	if got.Season != TimeAfterPentecost {
+		t.Errorf("EF season = %s want time-after-pentecost", got.Season)
+	}
+	// OF path unchanged for the same date
+	ofDay := Compute(d("2025-08-15"), DefaultSelection(), []Layer{layer})
+	if ofDay.Season != Ordinary {
+		t.Errorf("OF season = %s want ordinary", ofDay.Season)
+	}
+}