I am trying to add a radcombobox to every cell in the calender. but I get this error when I run the project:  
If I add a label as shown in one of your example it works fine but doesnt work with a radcombobox.
Page cannot be null. Please ensure that this operation is being performed in the context of an ASP.NET request.
 Protected Sub RadCalendar1_DayRender(ByVal sender As Object, ByVal e As Telerik.Web.UI.Calendar.DayRenderEventArgs)
            Dim ddl As New RadComboBox
            ddl.Items.Add(New RadComboBoxItem("", ""))
            ddl.Items.Add(New RadComboBoxItem("H", "H"))
            ddl.Items.Add(New RadComboBoxItem("V", "V"))
            ddl.Items.Add(New RadComboBoxItem("I", "I"))
            ddl.Items.Add(New RadComboBoxItem("T", "T"))
            e.Cell.Controls.Add(ddl)
    End Sub


Heres the calendar source
 <telerik:RadCalendar ID="RadCalendar1" Runat="server" MultiViewColumns="3"
                   SelectedDate="" ViewSelectorText="x" ondayrender="RadCalendar1_DayRender"
                   CssClass="RadCalendarMultiView RadCalendarMultiView_Default">
                   <dayoverstyle cssclass="rcHover" />
                   <fastnavigationstyle cssclass="RadCalendarMonthView RadCalendarMonthView_Default" />
                   <disableddaystyle cssclass="rcDisabled" />
                   <WeekendDayStyle CssClass="rcWeekend" />
                   <viewselectorstyle cssclass="rcViewSel" />
                   <SelectedDayStyle CssClass="rcSelected" />
                   <calendartablestyle cssclass="rcMainTable" />
                   <OtherMonthDayStyle CssClass="rcOtherMonth" />
                   <outofrangedaystyle cssclass="rcOutOfRange" />      
               </telerik:RadCalendar>


Please advice how can I show a rad combo box in there..

Hi Ropate,

I am afraid this scenario is not supported. RadComboBox is a complex control and cannot be nested inside a simple calendar cell due to design limitations. You can try adding a standard DropDownList instead. 

Greetings,
Marin
the Telerik team