Hi Giannip,
As far as I know, this function is not supported now. However, since it is an open source project we can modify its source code. We suggest that you'd better go through the CalendarBehavior.js. The easier way is to use Javascript, validator or server side logic to validate its selected date. For example,
<ajaxToolkit:CalendarExtender ID="CalendarExtender1" BehaviorID="myCEBID" runat="server" TargetControlID="TextBox1" Format="yyyy-MM-dd" PopupButtonID="Button1" OnClientDateSelectionChanged="onClientSelectDateChanged">
</ajaxToolkit:CalendarExtender>
function onClientSelectDateChanged(sender,args)
{
//do the validate work here.
}
Best regards,
Jonathan