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:

            MonthViewElement mve = (MonthViewElement)this.radCalendar1.CalendarElement.CalendarVisualElement;
            foreach (CalendarCellElement cell in mve.TableElement.Children)
            {
                if (cell.Row == 0)
                {
                    cell.DrawFill = true;
                    cell.BackColor = Color.Red;
                } 
            }