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. On 25 June 1967, with extensive previous service flying two tours in F-100s, Major Day was made the first commander of Detachment 1, 416th Tactical Fighter Squadron, 37th Tactical Fighter Wing based at Phu Cat Air Base. Under the project name "Commando Sabre", twin-seat USAF F-100Fs were evaluated as a Fast Forward Air Controller (Fast FAC) aircraft in high threat areas, given that F-4 Phantom II aircraft were in high demand for strike and Combat Air Patrol (CAP) roles. Using the call sign "Misty", the name of Day's favorite song, his detachment of four two-seat F-100Fs and 16 pilots became pioneer "Fast FACs") over Laos and North Vietnam. All Misty FAC crews were volunteers with at least 100 combat missions in Vietnam and 1,000 minimum flight hours. Tours in Commando Sabre were temporary and normally limited to four months or about 60 missions.