The ItemColumns is calculated based on the CalendarItemGroup.CurrentCalendarMode as well as other factors. When the mode is Days, the ItemColumns is based upon the number of visible days of the week which is affected by the ShowDisabledDaysOfWeek.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2