The JavaScript Calendar component fires the onEventMoved event handler when drag and drop event moving is completed (on drop), after the default action.
The default action is specified using the eventMoveHandling property - by default it is set to "Update" which means the event will be rendered at the new location.
See also the onEventMove event handler which is fired before the default action.
This event is available in the open-source version of the calendar component as well.
DayPilot.Calendar.onEventMoved(args);
dp.onEventMoved = (args) => { console.log("Moved: " + args.e.text()); };