You will need to manually post back your form with some client side javascript. Then you could just check the selected date to do what you want.
<cc1:CalendarExtender ID="CalendarExtender1" TargetControlID="txtDate" PopupButtonID="myButton" runat="server" OnClientDateSelectionChanged="dateChanged"></cc1:CalendarExtender>
function dateChanged() {
document.forms[0].submit();
}
Protected Sub Page_Load(ByVal sender As Object, ByVal e as EventArgs)
If txtDate.Text <> String.Empty Then DoSomething
End Sub
Private Sub DoSomething()
'do your work
End Sub