The MONTHDAYSTATE data type is a bitfield that holds the state of each day in a month. The data type is defined in Commctrl.h as follows:
typedef DWORD MONTHDAYSTATE, *LPMONTHDAYSTATE;
Each bit (0 through 30) 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.
This data type is used with the MCM_SETDAYSTATE message and the corresponding macro, MonthCal_SetDayState. When MONTHDAYSTATE values are used in reference to months shorter than 31 days, only the needed bits will be accessed.
This data type was first defined in Version 4.70 of Comctl32.dll.