To reproduce: 
1. Drag and drop RadCalendar to form
2. Set design time the AllowMultipleView property to true
3. Set design time value of the MultiViewRows and MultiViewColumns properties to 2
4. When save the form or run the project an exception is thrown. 

Workaround: 
Set the AllowMultipleView and MultiViewColumns properties with code.

To reproduce: 

1. When set the Disabled to true  of special day has not effect
this.radCalendar1.SpecialDays.Add(new RadCalendarDay(new DateTime(2014,7, 15)));
this.radCalendar1.SpecialDays[0].Disabled = true;

2. When set the Selected and Selectable to false, the special day is still can select and highlight
this.radCalendar1.SpecialDays[0].Selected = false;
this.radCalendar1.SpecialDays[0].Selectable = false;

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;
}

To reproduce:
- Add RadCalendar to a blank form and set its AllowMultipleSelect property to true.
- Select a date and deselect it right after that. You will notice that the background is not changed.
- The cell is actually deselected but it appears as selected since it is the focused cell and the styles are equal.

Workaround: Change the style for the focused state, which will allow the user to distinguish between focused and selected cells.

FIX. RadCalendar - Incorrect week numbering

Use the following code snippet:

radCalendar1.AllowMultipleView = true;
radCalendar1.MultiViewColumns = 1;
radCalendar1.MultiViewRows = 5;
radCalendar1.ShowRowHeaders = true;

CultureInfo ci = new CultureInfo("en-US");
CalendarWeekRule weekRule = CalendarWeekRule.FirstDay;
ci.DateTimeFormat.CalendarWeekRule = weekRule;
radCalendar1.Culture = ci;

Expected behavior: when the CalendarWeekRule is FirstDay, 01/01/2013 should be displayed as week 1

Use the following code snippet:

private void radCalendar1_ViewChanged(object sender, EventArgs e)
{
    this.radCalendar1.SpecialDays.Clear();

    RadCalendarDay day = new RadCalendarDay(new DateTime(2014,04,15));
    radCalendar1.SpecialDays.Add(day);
    PictureBox pictureBox = new PictureBox();
    pictureBox.SizeMode = PictureBoxSizeMode.StretchImage;
    pictureBox.Image = Properties.Resources.home;
    RadHostItem hostItem = new RadHostItem(pictureBox);
    day.TemplateItem = hostItem;
}

Clicking the navigation buttons does not navigate you to the respective month.

Workaround: assign the RadCalendarDay.TemplateItem before adding the day in the SpecialDays collection:
private void radCalendar1_ViewChanged(object sender, EventArgs e)
{
    this.radCalendar1.SpecialDays.Clear();

    RadCalendarDay day = new RadCalendarDay(new DateTime(2014,04,15));
    PictureBox pictureBox = new PictureBox();
    pictureBox.SizeMode = PictureBoxSizeMode.StretchImage;
    pictureBox.Image = Properties.Resources.home;
    RadHostItem hostItem = new RadHostItem(pictureBox);
    day.TemplateItem = hostItem; 
    radCalendar1.SpecialDays.Add(day);
}

Note: if the initial code is used in the ViewChanging event, clicking the navigation buttons leads to freezing the application.

RadCalendar should support month selection. I.e, the end-user should be able to press a single button, and all the cells from the current month should become selected.

Resolution: 

#1 This behavior could be achieved very easy with current API, so we are thinking  that there is no need to implement it in our RadCalendar as embedded functionality. 

For example: 

C#

            this.radCalendar1.AllowMultipleSelect = true;
            for (int i = 1; i <= DateTime.DaysInMonth(DateTime.Now.Year,DateTime.Now.Month); i++)
            {
                this.radCalendar1.SelectedDates.Add(new DateTime(DateTime.Now.Year, DateTime.Now.Month, i));
            }
VB

Me.radCalendar1.AllowMultipleSelect = True
For i As Integer = 1 To DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month)
Me.radCalendar1.SelectedDates.Add(New DateTime(DateTime.Now.Year, DateTime.Now.Month, i))
Next

#2 You can set ShowOtherMonthsDays to false and use ViewSelector. 

For example: 

C# 
this.radCalendar1.AllowMultipleSelect = true;
this.radCalendar1.AllowViewSelector = true;
this.radCalendar1.ShowViewSelector = true;
this.radCalendar1.ShowOtherMonthsDays = false;
VB
Me.radCalendar1.AllowMultipleSelect = True
Me.radCalendar1.AllowViewSelector = True
Me.radCalendar1.ShowViewSelector = True
Me.radCalendar1.ShowOtherMonthsDays = False

Presently RadCalendar does not support Persian calendar style.

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

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

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

IMPROVE. RadCalendar - expose the RefreshVisuals method of the CalendarElement

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

When you click on a Navigation Button with the mouse in RadCalendar and hold the button down, then it scrolls automatically to the next/previous views very fast. Same behavior should be supported by the Fast Navigation Buttons.

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;

When a RadRichTextBox control is added as a child to RadCalendar, the Space and Enter key input is stolen by the calendar.

Workaround is available here: http://www.telerik.com/community/forums/getting-visible-dates-from-radcalendar

The border of the special days in RadCalendar does not change on mouse leave.

When RadCalendar is docked inside a panel which is docked in a form and the calendar is in MultiMonth view, the month headers are not displayed properly.