BCGControlBar Pro for MFC
UINT BCGM_CALENDAR_ON_SELCHANGED

This message is sent to owner of calendar bar control when selected date has been changed.

Parameters
wParamThis parameter is not used.
lParama handle to calendar bar control.
Returns
The return value is ignored.

You should process this message to receive notifications from CBCGPCalendarBar objects when selection (selected date) has been changed.