DAYNAME(day-num)
day-num: The given value. If day-num is a number value, it must be in the range 1 to 7 (the decimal portion, if any, is ignored).
Examples
If B1 contains the date/time value August 2, 1979 06:30:00, C1 contains 10/16/2008 formatted as text, and D1 contains 6:
=DAYNAME(B1) returns Thursday.
=DAYNAME(C1) returns Thursday.
=DAYNAME(D1) returns Friday.
=DAYNAME("12/29/1979") returns Saturday.