BCGSuite for MFC
BOOL CBCGPCalendar::UnselectDate ( const COleDateTime &  date,
BOOL  bNotify = TRUE,
BOOL  bRedraw = TRUE 

Makes the specified date unselected.

Use this method to remove selection from the specified day. If bNotify is TRUE, the calendar calls OnSelectionChanged() (which in turn sends out a notification message BCGM_CALENDAR_ON_SELCHANGED).

dateSpecifies the date.
bNotifyIf TRUE, the calendar will notify the owner about selection changes.
bRedrawIf TRUE, the calendar will be redrawn to reflect the current selection state.