From February 1953 to August 1955 during the Korean War, Day served two tours as a fighter-bomber pilot, flying the Republic F-84 Thunderjet in the 559th Strategic Fighter Squadron. Promoted to captain, he decided to make the Air Force a career and was augmented into the Regular Air Force. He was assigned to the 55th Fighter Bomber Squadron. He then trained to fly the F-100 Super Sabre in 1957 while stationed at Royal Air Force Wethersfield in the United Kingdom through June 1959. It was during this time that he had to bail out of a jet fighter without a parachute, becoming the first person ever to live through such a feat. According to Day, a pine tree cushioned his fall.