So I just updated a client from FullCalendar like 1.6.4 to the latest 2.1.1 and the problem is jquery UI integration was removed from FullCalendar and I have some custom code to drag events from FullCalendar to an outside droppable div which worked in the old version because events where initiated as jquery ui draggable I guess but with the latest version dragged elements aren't recognized by the jquery UI droppable div as valid draggable/droppable elements.
Now I can get around this somewhat by loading jquery ui draggable and calling $('.fc-event').draggable() now the droppable div will be recognize by the droppable div as an acceptable droppable item BUT now dragging and dropping events on the calendar gets messed up because if we go to move an event from say one day to another then the event eventDrop should fire but instead since the events are initialized with jquery ui draggable and the calendar also accepts jquery ui draggable elements, the drop event is fired instead.
So now I'm not sure whats the easiest way to fix this problem...
Ideally I'd like to keep fullcalendar the way it is and somehow make the jquery ui droppable recognized calendar events as valid droppables.
I had the same problem with the fullcalendar. As I was trying to drag an event to a trash (outside div) I think the easiest way to do this is using this solution.
Remove Elements from fullcalendar (by dragging to trash can)
I would continue the study to understand why events from calendar aren't recognized by droppable, but I made some adaptations in the solution proposed in the link above and it works fine to me.
If I were you, I would take this way to solve any problem related to dragging events calendar to outside divs.