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;

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

Note: the issue is not reproduced consistently.

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);

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();

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);

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:
- 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.

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

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;
}

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.