Tuesday.
21 September 1971 was a Tuesday.
August 21 1971 was a Saturday.
May 21, 1971 fell on a Friday.
April 21, 1971 fell on a Wednesday.
December 21 1950 was on a Thursday.
21 December 1995 was a Thursday.
December 21 1951 was a Friday.
December 21 2012 will be a Friday.
December 21, 1962 was a Friday.
December 21, 2002 (Saturday).
December 21 2010 was a Tuesday.
December 21, 1991 was a Saturday.