eventRender
Triggered while an event is being rendered.
function( event, element, view ) { }
event is the Event Object that is attempting to be rendered.
element is a newly created jQuery <div> that will be used for rendering.
It has already been populated with the correct time/title.
The eventRender callback function can modify element, return a brand new DOM element that will be used for rendering instead, or it can return false, which will prevent the event from being rendered at all.
eventRender is a great way to attach other jQuery plugins to event elements, such as a qTip tooltip effect:
$('#calendar').fullCalendar({
  events: [
    {
      title: 'My Event',
      start: '2010-01-01',
      description: 'This is a cool event'
    }
    // more events here
  ],
  eventRender: function(event, element) {
    element.qtip({
      content: event.description
    });
  }
});
Note that description is a non-standard Event Object field, which is allowed.