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;

HeaderWidth and HeaderHeight properties should be made RadProperties and should be declared at RadCalendarElement. This will allow our users to set these properties in theme.

Meanwhile, you can workaround this by resetting the property every time you change the number of months that RadCalendar displays. This is shown in the next code snippet:

private void radPageViewPage2_ClientSizeChanged(object sender, EventArgs e)
{
    int numViews = this.radPageViewPage2.Size.Height / 240 + 1;
    radCalendar1.MultiViewRows = numViews;
    radCalendar1.ShowFastNavigationButtons = true;
    radCalendar1.ShowFastNavigationButtons = false;
}

The following line does not move MultiMonthView to the specific date:  

radCalendar1.CalendarElement.View.ViewStartDate = new DateTime(DateTime.Now.Year, 1, 1);


Workaround: use RadCalendar's FocusedDate property instead:

radCalendar1.FocusedDate = new DateTime(2014, 1, 1);

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.

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

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.

To reproduce: 

CultureInfo culture = new CultureInfo("en-US");
culture.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Monday;
culture.DateTimeFormat.CalendarWeekRule = CalendarWeekRule.FirstFourDayWeek;
this.radCalendar1.ShowRowHeaders = true;
this.radCalendar1.Culture = culture;

To reproduce:
public Form1()
{
    InitializeComponent();
    this.radCalendar1.RangeMinDate = new DateTime(2014, 1, 1);
    this.radCalendar1.RangeMaxDate = new DateTime(2017, 7, 17);
    this.radCalendar1.FocusedDate = new DateTime(2014, 2, 4);
    this.radCalendar1.AllowFastNavigation = true;
    this.radCalendar1.FastNavigationStep = 1; 
    this.radCalendar1.AllowMultipleView = true;
    this.radCalendar1.MultiViewColumns = 4;
    this.radCalendar1.MultiViewRows = 3;
    this.radCalendar1.CurrentViewRow = 0;  
    this.radCalendar1.CurrentViewColumn = 1;  
}

The user expects when fast navigating forward, the view to be shifted with one year = 12 months. However, each next forward fast navigation moves with 11 months. 

Workaround: 

            this.radCalendar1.CurrentViewRow = 0;  
            this.radCalendar1.CurrentViewColumn = 0;
            this.radCalendar1.CalendarElement.PreviousButton.PerformClick();

Currently, you can use reflection in order to access the properties of the fast navigation popup. 

Workaround: 
this.radCalendar1.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Popup;

CalendarNavigationElement a = this.radCalendar1.CalendarElement.CalendarNavigationElement;
FieldInfo fi = typeof(CalendarNavigationElement).GetField("dropDown", BindingFlags.NonPublic | BindingFlags.Instance);
RadDateTimePickerDropDown dropDown = (RadDateTimePickerDropDown)fi.GetValue(a);
dropDown.PopupOpening += new RadPopupOpeningEventHandler(dropDown_PopupOpening);

Please refer to the attached sample project and run it with RadForm2.

Note: the issue is not reproduced consistently.