asp.net AJAX Calendar control validation

Dear All,

               I have a problem in validating an AJAX Calande Control. I have an ajax calendar control as below.

<cc1:CalendarExtender ID="CalendarExtenderTXT_FromDate" runat="server" PopupButtonID="IMG_1" Format="MM/yyyy"  OnClientHidden="onCalendarHidden"  OnClientShown="onCalendarShown" BehaviorID="calendar1" Enabled="True" TargetControlID="TXT_FromDate" PopupPosition="BottomRight" ></cc1:CalendarExtender>

now i have to check that from current month to lessthen three month values can be selected . ie current month is SEPT it should allow to select month till "JULY"

How can i do it?

Any One can help? It's urgent.

Thanks in advance.