diff --git a/.changeset/rare-pens-notice.md b/.changeset/rare-pens-notice.md
new file mode 100644
index 00000000..6376635a
--- /dev/null
+++ b/.changeset/rare-pens-notice.md
@@ -0,0 +1,5 @@
+---
+'@when/worker': patch
+---
+
+better support for caldav all day busy events when calculating availability
diff --git a/packages/calendar/src/parse.test.ts b/packages/calendar/src/parse.test.ts
index b8a78951..5538ecdf 100644
--- a/packages/calendar/src/parse.test.ts
+++ b/packages/calendar/src/parse.test.ts
@@ -168,3 +168,58 @@ END:VEVENT
 END:VCALENDAR`;
 	expect(parseBusyEvents(cancelled)).toEqual([]);
 });
+
+test('parses all-day event with explicit DTEND', () => {
+	const alldayWithEnd = `BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//test//EN
+BEGIN:VEVENT
+UID:allday-end@test
+DTSTAMP:20260101T000000Z
+DTSTART;VALUE=DATE:20260415
+DTEND;VALUE=DATE:20260416
+SUMMARY:All day with DTEND
+END:VEVENT
+END:VCALENDAR`;
+	const busy = parseBusyEvents(alldayWithEnd);
+	expect(busy).toHaveLength(1);
+	expect(busy[0].uid).toBe('allday-end@test');
+	expect(busy[0].start.toString()).toBe('2026-04-15T00:00:00Z');
+	expect(busy[0].end.toString()).toBe('2026-04-16T00:00:00Z');
+});
+
+test('parses single-day all-day event without DTEND (RFC 5545 default 1 day duration)', () => {
+	const alldayNoEnd = `BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//test//EN
+BEGIN:VEVENT
+UID:allday-noend@test
+DTSTAMP:20260101T000000Z
+DTSTART;VALUE=DATE:20260415
+SUMMARY:All day single day
+END:VEVENT
+END:VCALENDAR`;
+	const busy = parseBusyEvents(alldayNoEnd);
+	expect(busy).toHaveLength(1);
+	expect(busy[0].uid).toBe('allday-noend@test');
+	expect(busy[0].start.toString()).toBe('2026-04-15T00:00:00Z');
+	expect(busy[0].end.toString()).toBe('2026-04-16T00:00:00Z');
+});
+
+test('parses timed event without DTEND or DURATION (0 duration)', () => {
+	const timedNoEnd = `BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//test//EN
+BEGIN:VEVENT
+UID:timed-noend@test
+DTSTAMP:20260101T000000Z
+DTSTART:20260415T140000Z
+SUMMARY:Zero duration
+END:VEVENT
+END:VCALENDAR`;
+	const busy = parseBusyEvents(timedNoEnd);
+	expect(busy).toHaveLength(1);
+	expect(busy[0].uid).toBe('timed-noend@test');
+	expect(busy[0].start.toString()).toBe('2026-04-15T14:00:00Z');
+	expect(busy[0].end.toString()).toBe('2026-04-15T14:00:00Z');
+});
diff --git a/packages/calendar/src/parse.ts b/packages/calendar/src/parse.ts
index b1ab7dce..e224dc36 100644
--- a/packages/calendar/src/parse.ts
+++ b/packages/calendar/src/parse.ts
@@ -30,7 +30,7 @@ function toBusy(event: IcsEvent): BusyEvent | null {
 
 	try {
 		const startMs = event.start.date.getTime();
-		const endMs = getEventEnd(event).getTime();
+		const endMs = eventEnd(event).getTime();
 		return {
 			uid: event.uid,
 			start: Temporal.Instant.fromEpochMilliseconds(startMs),
@@ -48,3 +48,20 @@ function toBusy(event: IcsEvent): BusyEvent | null {
 		return null;
 	}
 }
+
+function eventEnd(event: IcsEvent): Date {
+	if (event.end?.date) {
+		return event.end.date;
+	}
+	if (event.duration) {
+		return getEventEnd(event);
+	}
+	if (event.start.type === 'DATE') {
+		// All-day event without DTEND/DURATION lasts 1 full day per RFC 5545 §3.6.1
+		const d = new Date(event.start.date.getTime());
+		d.setUTCDate(d.getUTCDate() + 1);
+		return d;
+	}
+	// DATE-TIME event without DTEND/DURATION has 0 duration per RFC 5545 §3.6.1
+	return event.start.date;
+}