The format string specified for this property will override the property DayHeaderFormatString of base class on the first day of a month, and also on the first visible day of the WebMonthView (i.e., the first day shown of a trailing month).
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, 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