In September 1952, he completed The Basic School at Quantico, Virginia, and was transferred to Korea where he served with Company C, 1st Battalion, 7th Marines and the 1st Reconnaissance Company.

First Lieutenant Day served as the S-3 officer, Marine Corps Supply Center, Barstow, California, until July 1954, when he was transferred to Camp Pendleton, California, for duty as the commanding officer, Company C, Marine Corps Test Unit One. He was promoted to captain in December 1954. Capt Day remained at Camp Pendleton until May 1956, and was then assigned as Operations Officer of the Recruit Training Command, Marine Corps Recruit Depot, San Diego.

In September 1957, he was transferred to Okinawa and served as Commanding Officer, 4.2 Mortar Company, and later served as a battalion operations officer with the 9th Marines, 3d Marine Division. Returning stateside in December 1958, he was assigned as Instructor, Tactics Group, The Basic School, Quantico. He was promoted to major in August 1962 and attended the Amphibious Warfare School, also at Quantico.

Major Day was transferred to the 4th Marine Corps District in July 1963 and served as Inspector-Instructor, 43d Rifle Company, Cumberland, Maryland. In April 1966, Maj Day served his first tour in Vietnam as Commanding Officer, 1st Battalion, 9th Marines, 3d Marine Division. Returning to Camp Pendleton in June 1967, he was assigned as the Commanding Officer, 1st Battalion, 28th Marines, 5th Marine Division. He was promoted to lieutenant colonel in July 1967 and in January 1968, he was reassigned as Battalion Commander, 2d Infantry Training Regiment, Camp Pendleton.

Lieutenant Colonel Day served at Pearl Harbor, Hawaii, from July 1969 to June 1971 and attended the Army War College, Carlisle Barracks, Pennsylvania, from July 1971 to June 1972. After graduation, he served his second tour in Vietnam as Operations Officer, 9th Marine Amphibious Brigade, III Marine Amphibious Force. He was reassigned as Commanding Officer, Camp Fuji, Japan, in March 1973.

He was promoted to colonel in November 1973 and was transferred to Philadelphia for duty as Deputy Director, and later, Director, 4th Marine Corps District. He remained in that billet until 1 April 1976, when he was advanced to brigadier general. He assumed duties as Assistant Depot Commander, Marine Corps Recruit Depot, San Diego, in May 1976, and on 1 November 1977, he became Commanding General of the Depot, serving in that capacity until 11 March 1978.

On 29 April 1978, he was assigned duty as Deputy Director for Operations, J-3, NMCC, Joint Chiefs of Staff, Washington, D.C. During July 1979, BGen Day was assigned duty as the Assistant Division Commander, 1st Marine Division/Commanding General, 7th Marine Amphibious Brigade, Fleet Marine Force, Pacific, Camp Pendleton. He was promoted to major general on 1 August 1980, and assumed duty as the Commanding General, 1st Marine Division, and was ultimately assigned the additional duty as Commanding General, I MAF, on 1 July 1981. He served in that capacity until August 1982 when he was assigned duty as the Deputy Chief of Staff for Training, Headquarters, U.S. Marine Corps, Washington, D.C. In July 1984, he was assigned duty as the Commanding General, Marine Corps Base, Camp S. D. Butler/Deputy Commander, Marine Corps Bases, Pacific (Forward)/Okinawa Area Coordinator, Okinawa, Japan. He served in this capacity until his retirement on 1 December 1986. Upon his retirement, he was presented the Distinguished Service Medal for exceptionally meritorious service to the Government of the United States for duties while serving in his final duty station.