Thursday. Check out the related link I added.
September 15 1983 was a Thursday.
May 15, 1983 fell on a Sunday.
July 15, 1983 fell on a Friday.
December 15, 1977 was a Thursday.
15 December 1958 was a Monday.
15 December 1997 was a Monday.
15 December 1935 was on a Sunday.
December 15 2010 was a Wednesday.
December 15, 1936 was a Tuesday.
15 December 1980 was a Monday.
December 15 1993 was on a Wednesday.