David Kimura PRO said over 7 years ago on FullCalendar Events and Scheduling :

Yeah, I'd calculate it on the server side and then pass the blocked days through the JSON render request. On the client side, put in the results of the client side validation. Overall, you just want to make sure that you're aren't allowing a client to go through a whole event record creation, only to tell them something that you already knew and could have told them from the beginning; that enrollment for that day is already filled.