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.

[3] 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.

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.

[3] 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.

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.

Day was on his 65th mission into North Vietnam and acting as check pilot for Captain Corwin M. "Kipp" Kippenhan, who was upgrading to aircraft commander.

Within 2 miles (3 kilometers) of the U.S. Marine firebase at Con Thien and after 12 to 15 days of evading, he was captured again, this time by a Viet Cong patrol that wounded him in the leg and hand with gunfire.

In December 1967, Day shared a cell with Navy Lieutenant Commander and future senator and presidential candidate John McCain.

Air Force Major Norris Overly nursed both back to health, and McCain later devised a makeshift splint of bamboo and rags that helped heal Day's seriously atrophied arm.

On 4 March 1976, President Gerald Ford awarded Day the Medal of Honor for his personal bravery while a captive in North Vietnam.

He underwent conversion training to the F-4 Phantom II and was appointed vice commander of the 33rd Tactical Fighter Wing at Eglin Air Force Base, Florida.

"[11] After being passed over for nomination to brigadier general, Day retired from active duty in 1977 to resume practicing law in Florida.

Day was posthumously advanced to the rank of brigadier general during a Heritage to Horizons summer concert series at the Air Force Memorial in Arlington, Virginia, on June 8, 2018.

The ceremony was presided over by the Chief of Staff of the Air Force, General David L. Goldfein, and attended by his widow, Doris Day.

The posthumous advancement of Day was introduced by former prisoner of war cell mate John McCain and was directed by the 2017 National Defense Authorization Act.

[13] Day was an active member of the Florida Republican Party, was involved in the 527 group Swift Vets and POWs for Truth,[14] and campaigned with John McCain in 2000 and 2008.

[15] In the months leading up to the 2004 U.S. presidential election, Day appeared in television advertisements—along with other members of the 527 group Swift Vets and POWs for Truth—decrying John Kerry's antiwar activities following his military service during the Vietnam War and declaring him "unfit" for service and of a "dishonest" disposition for comments and actions made by Kerry after the Vietnam War, including his testimony before Congress in Washington, D.C.[16] During a 2008 teleconference with reporters from the Miami Herald, Day made comments regarding John McCain's stance on the Iraq War, stating that "I don't intend to kneel, and I don't advocate to anybody that we kneel, and John [McCain] doesn't advocate to anybody that we kneel."

Also during this interview he sparked controversy by making a broad generalization about what some saw as an ideological divide between Islam and America: "The Muslims have said either we kneel, or they're going to kill us.

On 7 May 2010, Day Manor, a visiting officers quarters (VOQ) at Goodfellow Air Force Base, Texas, was dedicated in his honor.

Citation: On 26 August 1967, Colonel Day was forced to eject from his aircraft over North Vietnam when it was hit by ground fire.

After several unsuccessful attempts to signal U.S. aircraft, he was ambushed and recaptured by the Viet Cong, sustaining gunshot wounds to his left hand and thigh.

Colonel Day withstood this punishment and gave nothing of value to the Vietnamese, although he sustained many injuries and open wounds to his body.

Through his extraordinary heroism and willpower, in the face of the enemy, Colonel Day reflected the highest credit upon himself and the United States Air Force.