It was a Friday. See related link below.
Friday.
Friday
It was a Saturday.
September 21 2004 was a Tuesday.
September 21, 1972 was on a Thursday
September 21, 1961 was a Thursday.
21 November 1973 was a Wednesday.
February 21, 1973, was a Wednesday.
June 21 1973 was a Thursday.
April 21, 1973 fell on a Saturday.
October 21, 1973 was a Sunday.
September 21, 1985, was a Saturday.