MCM_SETFIRSTDAYOFWEEK message

Sets the first day of the week for a month calendar control. You can send this message explicitly or by using the MonthCal_SetFirstDayOfWeek macro.

wParam

Must be zero.

lParam

Value of type int representing which day is to be set as the first day of the week, where 0 is Monday, 1 is Tuesday, and so on.

Returns a DWORD value that contains two values. The high word is a BOOL value that is nonzero if the previous first day of the week did not equal LOCALE_IFIRSTDAYOFWEEK, or zero otherwise. The low word is an INT value that represents the previous first day of the week.

If the first day of the week is set to anything other than the default (LOCALE_IFIRSTDAYOFWEEK), the control will not automatically update first-day-of-the-week changes based on locale changes.