Day, who died Saturday, was among the United States' most highly decorated servicemen, having received nearly 70 medals and awards, The New York Times reported.
The colonel volunteered for duty in Vietnam with the U.S. Air Force in 1967. Later that year, his plane was shot down by North Vietnamese soldiers and he was taken captive.
Day managed to escape five days later, but was recaptured as he made his way to a Marine outpost. Day was taken to the Hoa Lo Prison, also known as the Hanoi Hilton, where he was held until March 14, 1973.
At the Hoa Lo Prison, Day shared a cell with McCain, who went on to become a U.S. senator from Arizona and 2008 Republican presidential candidate.
In McCain's memoir, "Faith of My Fathers," the senator remarked that Day "was a tough man, a fierce resister, whose example was an inspiration to every man who served with him."
On Sunday, McCain tweeted that Day was "my friend, my leader, my inspiration."
Day is survived by his wife Doris; his two sons, Steven and George Jr.; two daughters, Sandra Hearn and Sonja LaJeunesse; and 14 grandchildren.