I could not find a way for an event call when all events are rendered. Once all events are rendered then I need iterating through the each week of the month and do some calculations on it.
It looks as this option is available version 3 but not in version 5
Maybe you should have a look at https://fullcalendar.io/docs/eventsSet
Please note that this callback is also called during the init phase (So its first call MAY NOT be the one you are waiting for)
In my case, I had to use a boolean to know if this callback has been triggered after an AJAX getEvents has been completed.