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

(calendar-day-name DATE &optional ABBREV ABSOLUTE)

Return a string with the name of the day of the week of DATE.
DATE should be a list in the format (MONTH DAY YEAR), unless the
optional argument ABSOLUTE is non-nil, in which case DATE should
be an integer in the range 0 to 6 corresponding to the day of the
week. Day names are taken from the variable `calendar-day-name-array',
unless the optional argument ABBREV is non-nil:
`header' means to use `calendar-day-header-array';
t to use `calendar-day-abbrev-array'.