calendar-day-of-week

calendar-day-of-week is a compiled Lisp function in `calendar.el'.

(calendar-day-of-week DATE)

Return the day-of-the-week index of DATE, 0 for Sunday, 1 for Monday, etc.
DATE is a list of the form (month day year). A negative year is
interpreted as BC; -1 being 1 BC, and so on.