This function will return the day of the week ("Sunday", "Monday", "Tuesday", etc) for any date back to January 1st, 0001 AD.
This function is affected by the changeover date for the Gregorian calendar as specified in Preferences (i.e. either 1582 or 1752).
=DayOfWeek(%INDI.BIRT.DATE%)
=DayOfWeek(DateAt(2008,1,1))