An application sends a ZGM_GETREGDATEDOW message to get the day of the week of the date in the internal date register.
ZGM_GETREGDATEDOW
wParam = 0; //not used; must be 0
lParam = 0; //not used; must be 0
If the date in the internal date register is valid, this message returns the day of the week of that date. Otherwise, the message returns -1.
The following values are returned based on the day of the week: