Presently RadCalendar does not support Persian calendar style.

Allow RadCalendar to have multiview based on years or months.

Resolution: With our latest release(Q2 2014) we are introducing new navigation mode that has very similar behavior the described one. To take the advantages of new mode you need only to set the HeaderNavigationMode property to "Zoom":
For example : this.radCalendar1.HeaderNavigationMode = HeaderNavigationMode.Zoom;

FIX. RadCalendar - add NewDate and OldDate to the SelectionEventArgs of the SelectionChanging event

Add functionality to customize navigation steps in multiple view of the RadCalendar.

It will be nice if RadCalendar can show a popup list containing years. Currently, it supports only a list of months.

RangeMinDate and RangeMaxDate properties of RadCalendar are not taken under consideration when a date is being selected.

Adding the following method CalendardDayCollection.AddRange(IEnumerable Of Date) is an easy way to populate the collection

Run the attached sample project and try entering "1400/12/30".

            CultureInfo info = new CultureInfo("fa-Ir");

            this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
            this.radDateTimePicker1.CustomFormat = "yyyy/MM/dd";
            this.radDateTimePicker1.Culture = info;

The following exception occurs: 

   at System.Globalization.PersianCalendar.ToDateTime(Int32 year, Int32 month, Int32 day, Int32 hour, Int32 minute, Int32 second, Int32 millisecond, Int32 era)
   at System.Globalization.Calendar.ToDateTime(Int32 year, Int32 month, Int32 day, Int32 hour, Int32 minute, Int32 second, Int32 millisecond)
   at Telerik.WinControls.UI.ArabicMaskDateTimeProvider.KeyPress(Object sender, KeyPressEventArgs e)
   at Telerik.WinControls.UI.RadMaskedEditBoxElement.TextBoxItem_KeyPress(Object sender, KeyPressEventArgs e)
   at System.Windows.Forms.KeyPressEventHandler.Invoke(Object sender, KeyPressEventArgs e)
   at Telerik.WinControls.RadItem.OnKeyPress(KeyPressEventArgs e)
   at Telerik.WinControls.UI.RadTextBoxItem.TextBoxControl_KeyPress(Object sender, KeyPressEventArgs e)
   at System.Windows.Forms.Control.OnKeyPress(KeyPressEventArgs e)
   at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
   at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
   at System.Windows.Forms.Control.WmKeyChar(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
   at System.Windows.Forms.TextBox.WndProc(Message& m)
   at Telerik.WinControls.UI.HostedTextBoxBase.WndProc(Message& message)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at DateTimePickerPersianCalendar.Program.Main() in c:\Projects\DateTimePickerPersianCalendar\DateTimePickerPersianCalendar\Program.cs:line 17
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

By default, if you need to format the cells in RadCalendar, the ElementRender event is an appropriate solution. However, it doesn't process the row/column header cell elements illustrated in the attached screenshot.

Workaround:

To reproduce: use the following code snippet:

            radCalendar1.AllowMultipleSelect = true;
            radCalendar1.ShowRowHeaders = true;
            radCalendar1.AllowRowHeaderSelectors = true;
 If you select several cells, they all are selected. Then, click the header row cell. You will notice that the cells from this row are selected but all previously selected cells are cleared. The attached gif file illustrates the behavior. 

Workaround: use the MouseDown event to store the currently selected cells and then on MouseUp restore the missing selection:

public class MyCalendar : RadCalendar
{
    List<DateTime> selectedDays = new List<DateTime>();

    protected override void OnMouseDown(MouseEventArgs e)
    {
        CalendarCellElement cell = this.ElementTree.GetElementAtPoint(e.Location) as CalendarCellElement;
        if (cell != null)
        {
            bool isHeader = (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty);
            if (isHeader)
            {
                selectedDays.Clear();
                foreach (DateTime date in this.SelectedDates)
                {
                    selectedDays.Add(date);
                }
            }
        }
        base.OnMouseDown(e);
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);
        foreach (DateTime date in selectedDays)
        {
            if (!this.SelectedDates.Contains(date))
            {
                this.SelectedDates.Add(date);
            }
        }
    }

    public override string ThemeClassName  
    { 
        get 
        { 
            return typeof(RadCalendar).FullName;  
        }
    }
}

To reproduce: use the following code snippet:

public Form1()
{
    InitializeComponent();

    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.MultiViewColumns = 4;
    this.radCalendar1.AllowMultipleSelect = true;
    this.radCalendar1.ShowRowHeaders = true;
    this.radCalendar1.ShowColumnHeaders = true;
    this.radCalendar1.AllowColumnHeaderSelectors = true;
    this.radCalendar1.AllowRowHeaderSelectors = true;
    this.radCalendar1.ShowViewSelector = true;
    this.radCalendar1.AllowViewSelector = true;

  
    this.radCalendar1.RangeMinDate = new DateTime(2014, 3, 3);
    this.radCalendar1.RangeMaxDate = new DateTime(2014, 7, 27);
    this.radCalendar1.FocusedDate = new DateTime(2014, 5, 5);
}

Please refer to the attached gif file.

Workaround: 
private void Calendar_MouseDown(object sender, MouseEventArgs e)
{
    CalendarCellElement cell = this.radCalendar1.CalendarElement.ElementTree.GetElementAtPoint(e.Location) as CalendarCellElement;
    if (cell != null && cell.Date < this.radCalendar1.RangeMinDate && (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty) == true)
    {
        foreach (MonthViewElement monthView in this.radCalendar1.CalendarElement.CalendarVisualElement.Children[0].Children.Last().Children)
        {
            {
                foreach (CalendarCellElement monthCell in monthView.TableElement.Children)
                {
                    if (monthCell.Date.Month == cell.Date.Month && monthCell.Date.Year == cell.Date.Year &&
                        this.radCalendar1.RangeMinDate <= monthCell.Date)// &&
                       // (bool)cell.GetValue(CalendarCellElement.IsHeaderCellProperty) == false)
                    {
                        if (char.IsLetter(cell.Text[0]))
                        {
                            //perform column selection
                            if (monthCell.ControlBoundingRectangle.X == cell.ControlBoundingRectangle.X)
                            {
                                this.radCalendar1.SelectedDates.Add(monthCell.Date);
                            }
                        }
                        else
                        {
                            //perform row selection
                            if (monthCell.ControlBoundingRectangle.Y == cell.ControlBoundingRectangle.Y)
                            {
                                this.radCalendar1.SelectedDates.Add(monthCell.Date);
                            }
                        }
                    }
                }
            }
        }
    }
}

ADD. RadCalendar's TodayButton does not accept shortcuts.

IMPROVE. RadCalendar - expose the RefreshVisuals method of the CalendarElement

To reproduce: use the following code snippet:

public Form1()
{
    InitializeComponent();
    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Zoom;
}

The event should fire so one can customize the appearance on loading.

Although RadCalendarDay has a Tooltip property, it does not show any tooltips.

To reproduce: select 01/01/0001 and click the previous button

The month selector of RadCalendar should fire Calendar.Navigating and Calendar.Navigated events.