<script type="text/javascript">
    function OnValueChanging(sender, args) {
        // Validate the "newValue" using Regex or any other means.
        // If this is valid it will return a date object type
        // When valid just fall through and let it select. 
        var inputValue = args.get_newValue();
        // Assume is true for input value from popup
        var ValidDateType = inputValue instanceof Date;

        // If it's an invalid date type
        if (ValidDateType == false) {
            // Validate against REGEX
            var IsValidDate = new RegExp('^((0[1-9])|(1[0-2]))\/(\d{4})$\/gm').test(inputValue);

            // 
            if (IsValidDate === false) {
                // Cancel the event (optional) 
                // This will revert the value
                //args.set_cancel(true);

                // Set invalid flag for the control
                sender.set_invalid(true);

                // Implement some more lines of code that will handle additional tasks
                DisplayVadlidity(true, inputValue);
            } else {
                DisplayVadlidity(false);
            }
        } else {
            DisplayVadlidity(false);
        }
    }

    function DisplayVadlidity(bool, dateValue) {
        if (bool) {
            var lbl = document.getElementById('<%= Label1.ClientID %>');
            lbl.innerText = "The value \"" + dateValue + "\" has an incorrect format.\nValue must have the format of \"MM/yyyy\".";
        } else {
            var lbl = document.getElementById('<%= Label1.ClientID %>');
            lbl.innerText = "";
        }
    }
</script>
<div>
    <telerik:RadMonthYearPicker EnableAriaSupport="true" EnableKeyboardNavigation="true" ID="from_date"
        runat="server"
        Width="150px" ToolTip="" Height="40px">
        <DateInput ID="fd_input" DateFormat="MM/yyyy" runat="server" Enabled="true">
            <ClientEvents OnValueChanging="OnValueChanging" />

        </DateInput>
    </telerik:RadMonthYearPicker>
    <br />
    <asp:Label ID="Label1" runat="server" ForeColor="Red"></asp:Label>
</div>