A month calendar control that uses the MCS_DAYSTATE style supports day states. The control uses day state information to determine how it draws specific days within the control. Day state information is expressed as a 32-bit data type, MONTHDAYSTATE. Each bit in a MONTHDAYSTATE bit field, from 1 through 31, represents the state of a day in a month. If a bit is on, the corresponding day will be displayed in bold; otherwise it will be displayed with no emphasis.
An application can explicitly set day state information by sending the MCM_SETDAYSTATE message or by using the corresponding macro, MonthCal_SetDayState. Additionally, month calendar controls that use the MCS_DAYSTATE style send MCN_GETDAYSTATE notification messages to request day state information.