In AjaxToolKit calendar control, StartDate and EndDate properties helped for the user defined date range.
<ajaxToolkit:Calendar runat="server"
TargetControlID="Date1"
CssClass="ClassName"
Format="MMMM d, yyyy"
PopupButtonID="Image1" />
StartDate - Indicates start date for range that available for selection.
EndDate - Indicates end date for range that available for selection.