<asp:TextBox ID="Datum" runat="server"></asp:TextBox>
<asp:CustomValidator ID="CustomValidator1" runat="server"
     ControlToValidate="Datum" Display="Dynamic"
     ErrorMessage="CustomValidator" ValidateEmptyText="True"
     Text="Bitte geben Sie ein gültiges Datum im Format TT.MM.JJJJ ein."
     OnServerValidate="CheckValidDate_ServerValidate">
</asp:CustomValidator>
<ajaxToolkit:CalendarExtender
     ID="CalendarExtender1" TargetControlID="Datum"
     runat="server" Format="dd.MM.yyyy">
</ajaxToolkit:CalendarExtender>

protected void CheckValidDate_ServerValidate(object source, ServerValidateEventArgs args)
    {
        try
        {
            DateTime.ParseExact(args.Value, "d", CultureInfo.CreateSpecificCulture("de-DE"));
            args.IsValid = true;
        }
        catch (Exception)
        {
            args.IsValid = false;
        }
    }