This demo demonstrates how to disallow overlaping two appointments DailyCalendar.AllowOverlap Additionally you can create client script event invoked when an appointment will be moved which will be programmatically control drag & drop operations. This demo shows how to prohibit dragging appointments before 10 am. Notice, the all day events are used for overlapping appointment checking as well. That is reason why you cannot drag appointment in columns, where all day appointment exist. DailyCalendar.OnClientAppointmentDragged Move appointment around calendar surface.

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If you would look into a source code for this demo, click on the link.
For demonstration purposes all appointments displayed on a calendar are stored in ASP.NET session objects, so original values are restored immediately after a new session object is created.
A list of appointments and resources is intialized every time a new session is started.