The onEventDelete event fires when the user clicks the event delete icon which is displayed on the right side of calendar events. The "delete" icon is only visible if you enable event deleting using eventDeleteHandling property of the monthly calendar.
This event is fired before the event is actually removed from the calendar. You can cancel the removal using args.preventDefault(). See also the onEventDeleted event which is called after the event is removed.
DayPilot.Month.onEventDelete(args);