Monday.
May 31, 1965, was a Monday.
31 December 1965 was a Friday.
31 January 1965 was a Sunday.
August 31, 1965 was a Tuesday.
October 31, 1965 fell on a Sunday.
31 May 1958 was a Saturday.
May 31 1967 was a Wednesday.
May 31 1966 was a Tuesday.
May 31 1947 was a Saturday.
May 31, 1976 was a Monday.
May 31 2010 was a Monday.
May 31, 1952, was a Saturday.