 Enlisting in the USMC on 10 December 1942, Day served 30 months in the North Pacific during World War II as a member of a 5-inch gun battery with the 3rd Defense Battalion on Johnston Island, but he never saw combat.He was discharged (the first time) on 24 November 1945.On 11 December 1946, Day joined the Army Reserve, serving until 10 December 1949.