These docs are for an old release.
Info on upgrading to v4
eventDragStart
Triggered when event dragging begins.
function( event, jsEvent, ui, view ) { }
event is an Event Object that hold the event’s information (date, title, etc).
jsEvent holds the jQuery event with low-level information such as click coordinates.
ui holds an empty object. Before version 2.1, the jQuery UI object.
view holds the current View Object.