Why can’t I select a day or time on the birthday party calendar?

Our birthday calendar is a live, up-to-date calendar which shows all our party slots for each weekend. If the calendar date is red and able to be clicked, there are still times available on that date. If it is grey and unable to be clicked, all parties on that day have been booked. Once you click the date, the party times that have not yet been booked will be red and able to be selected. If any party time is grey and unable to be clicked, then that party slot has been booked.