use proper formating strings... to format the date of AjaxCalendarControls....
see the link below for AjaxCalendarControl demonstration..
http://www.asp.net/ajax/ajaxcontroltoolkit/Samples/Calendar/Calendar.aspx
and to give the value for "Format" property of string have a look on the Format Strings in the link below.............
http://msdn.microsoft.com/en-us/library/bb79761a-ca08-44ee-b142-b06b3e2fc22b.aspx
<cc1:calendarextender id="CalendarExtender1" runat="server" xmlns:cc1="#unknown">
Enabled="True" PopupButtonID="Image3" Format="MMMM d, yyyy"
TargetControlID="txt"></cc1:calendarextender>