diff --git a/internal/service/week_service.go b/internal/service/week_service.go
index 5634861..f94d288 100644
--- a/internal/service/week_service.go
+++ b/internal/service/week_service.go
@@ -65,8 +65,11 @@ func weekDayKeys(weekKey string, tz *time.Location) ([]string, error) {
 	// Find the Monday of that ISO week.
 	// Jan 4 is always in week 1 of its year.
 	jan4 := time.Date(year, time.January, 4, 0, 0, 0, 0, tz)
+	// (weekday+6)%7 gives days-since-Monday (Mon=0 … Sun=6), avoiding the
+	// sign issue when Weekday()==0 (Sunday) with the naive subtraction.
+	daysSinceMonday := int(jan4.Weekday()+6) % 7
 	_, jan4Week := jan4.ISOWeek()
-	monday := jan4.AddDate(0, 0, -int(jan4.Weekday()-time.Monday)+(week-jan4Week)*7)
+	monday := jan4.AddDate(0, 0, -daysSinceMonday+(week-jan4Week)*7)
 
 	keys := make([]string, 7)
 	for i := 0; i < 7; i++ {
diff --git a/internal/service/week_service_test.go b/internal/service/week_service_test.go
index a52781f..3224997 100644
--- a/internal/service/week_service_test.go
+++ b/internal/service/week_service_test.go
@@ -34,18 +34,30 @@ func newFullServices(t *testing.T) (*service.EntryService, *service.DayService,
 
 func TestWeekDayKeys(t *testing.T) {
 	tz, _ := time.LoadLocation("UTC")
-	keys, err := service.WeekDayKeysExported("2024-W03", tz)
-	if err != nil {
-		t.Fatal(err)
+
+	cases := []struct {
+		weekKey string
+		wantMon string
+		wantSun string
+	}{
+		{"2024-W03", "2024-01-15", "2024-01-21"}, // Jan 4 = Thursday
+		{"2026-W18", "2026-04-27", "2026-05-03"}, // Jan 4 = Sunday — was broken
+		{"2023-W01", "2023-01-02", "2023-01-08"}, // Jan 4 = Wednesday
 	}
-	if len(keys) != 7 {
-		t.Fatalf("expected 7 keys, got %d", len(keys))
-	}
-	if keys[0] != "2024-01-15" {
-		t.Errorf("expected Monday 2024-01-15, got %s", keys[0])
-	}
-	if keys[6] != "2024-01-21" {
-		t.Errorf("expected Sunday 2024-01-21, got %s", keys[6])
+	for _, tc := range cases {
+		keys, err := service.WeekDayKeysExported(tc.weekKey, tz)
+		if err != nil {
+			t.Fatalf("%s: %v", tc.weekKey, err)
+		}
+		if len(keys) != 7 {
+			t.Fatalf("%s: expected 7 keys, got %d", tc.weekKey, len(keys))
+		}
+		if keys[0] != tc.wantMon {
+			t.Errorf("%s: Monday want %s, got %s", tc.weekKey, tc.wantMon, keys[0])
+		}
+		if keys[6] != tc.wantSun {
+			t.Errorf("%s: Sunday want %s, got %s", tc.weekKey, tc.wantSun, keys[6])
+		}
 	}
 }