Why can't customers book a certain date?

The usual causes — advance-booking window, blocked dates, closures, covers caps or a minimum lead time.

The most common reason is your advance-booking window: customers can only book up to your Maximum advance booking days ahead (Settings → Bookings → Booking rules — and your plan caps it at 5 / 14 / 30 / 365 days on Free / Plus / Pro / Complete), and a Minimum advance booking in hours can also rule out near-term dates. Otherwise, check Settings → Venue → Hours: the date may be marked Closed by your standard, seasonal or special hours, or listed under Online booking blocked dates (which stops online bookings even though staff can still book manually). Finally, a full day — every suitable table taken, or your Maximum covers per time slot reached — leaves nothing for customers to pick.

Related: How far ahead (and how late) can customers book?