 Day was assistant professor of aerospace science at the Air Force ROTC detachment at Saint Louis University in St. Louis, Missouri, from June 1959 to August 1963.Anticipating retirement in 1968 and now a major, Day volunteered for a tour in South Vietnam and was assigned to the 31st Tactical Fighter Wing at Tuy Hoa Air Base in April 1967.At that time, he had more than 5,000 flying hours, with 4,500 of them in fighters.