Put more than two RadDatePicker with single SharedCalendar on page. some RadDatePicker are in side of UpdatePanel with a button in it. when I press that button RadDatePicker behaviour is strange. 
RadDatePicker inside UpdatePanel not opening Calendar popup while RadDatePicker outside UpdatePanel opens calendar popup but gives javascript error on any date selection. the javascript error is "Unable to get property '_selectedDates' of undefined or null reference".
It looks like it is destroying shared calendar object on partial postback.
Please find below sample code to replicate the issue. after clicking "Submit" button, it doesn't open calendar popup for "Date 2" and for "Date 1" it opens but doesn't allow to select date and gives JavaScript error.

----------------------
<asp:ScriptManager ID="sc1" runat="server" />
<telerik:RadCalendar ID="dtCal" runat="server" EnableMultiSelect="false" />
Date 1:
<telerik:RadDatePicker ID="dt1" Width="103px" runat="server" SharedCalendarID="dtCal" />
<hr />
<asp:UpdatePanel runat="server" ID="up">
	<ContentTemplate>
		Date 2:
		<telerik:RadDatePicker ID="dt2" Width="103px" runat="server" SharedCalendarID="dtCal" />
		<br />
		<asp:Button runat="server" ID="btnDT" Text="Submit" />
	</ContentTemplate>
</asp:UpdatePanel>
----------------------

When i add all controls in UpdatePanel it works properly. but Actual code is much more complex than above sample with too many lines of javascript code and many date pickers on single page and simply all Calendar and DatePicker controls can not be moved in UpdatePanel.

RadMonthYearPicker is missing form the SkinManager's ControlTypeToApplyskin collection

-> When DatePicker renders for the first time, it applies the CSS according to the selected skin.
Reference Image: Comtrol in Filter Panel first time render.jpg

-> But as soon as I enter into an edit mode or insert than the CSS is not applied to the control and the design of the RadCalender is breaking somehow.
Reference Image: Control while Editing.jpg

-> Also entering into edit or insert break the design of the other RadCalender control placed in filter panel.
Reference Image: Control in Filter Panel.jpg

As I'm using RadDatePicker in my Grid,

In RadGrid for the first time, the image beside the calendar control renders as expected.

As soon as I enter into "InLine" edit mode there would be 2 images rendered beside the control.

And after clicking on cancel there is no change in the image beside the control.

Also attached the screenshot for the same.

Telerik.Web.UI. Version: 2013.2.611.40
Try the code below with a simple aspx page:
<asp:ScriptManager runat="server"/>
<telerik:RadDateTimePicker runat="server" ID="DatePicker" />

the code is:
		protected void Page_Load(object sender, EventArgs e)
		{
			DatePicker.SkipMinMaxDateValidationOnServer = true;
			DatePicker.MinDate = DateTime.Parse("01.01.2014");
			DatePicker.SelectedDate = DateTime.Parse("01.01.2013");
			DatePicker.SelectedDate = DateTime.Parse("01.01.2015");
		}

Actual result: 
Browser shows 01.01.2013 date marked in red but not 01.01.2015 as it supposes to show

When using min/max date the invalid dates have almost identical styling as the valid ones, this makes it hard for the user to see what dates are valid.

It would be nice of there was some way of telling them apart except by hovering over them.

Also when using "metro theme" the invalid date overrides the styling for dates belonging to "other month".

RadCalendar and RadDatePicker cannot pass the following check by https://achecker.ca/checker/index.php

WCAG 2.0 guideline
Section

1.3 Adaptable: Create content that can be presented in different ways (for example simpler layout) without losing information or structure.
Success Criteria 1.3.1 Info and Relationships (A) 

Check 245: Data table with more than one row/column of headers does not use id and headers attributes to identify cells.

Repair: Add id and headers attributes to table cells so they identify the cells that relate to the headers.

Error Line 597, Column 8:
<table id="ctl00_ContentPlaceholder1_RadDatePicker1_calendar_Top" class="rcMainTable" summary="Table …

Steps to reproduce

Go to https://achecker.ca/checker/index.php

Paste https://demos.telerik.com/aspnet-ajax/datepicker/accessibilityandinternationalization/wcag2.0andsection508accessibility/defaultcs.aspx
or 
https://demos.telerik.com/aspnet-ajax/calendar/accessibility-and-internationalization/wcag-2.0-and-section-508-accessibility/defaultcs.aspx

See the results -> 

Check 245: Data table with more than one row/column of headers does not use id and headers attributes to identify cells.
Repair: Add id and headers attributes to table cells so they identify the cells that relate to the headers.
Error Line 568, Column 8:
<table id="ctl00_ContentPlaceholder1_RadDatePicker1_calendar_Top" class="rcMainTable" summary="Table ...


=======================================

WORKAROUND:

$telerik.$("table.rcMainTable tbody th").each(function (idx, el) {
    var thId = $telerik.$(el).attr("id");
    $telerik.$(el).siblings("td").attr("header", thId);
})