Day was a two-time All-Southeastern Conference quarterback for the University of Mississippi (Ole Miss) Rebels, leading Ole Miss to SEC titles in 1954 and 1955 with a combination of running and passing skills.
Day led Ole Miss to a 26–5–1 record in three seasons while playing for coach Johnny Vaught. He was the MVP of the 1956 Cotton Bowl Classic after leading Ole Miss to a 14–13 upset victory over Texas Christian in the Rebels first major bowl game. After a late-game 25-yard scramble during the 1956 Cotton Bowl Classic game against Texas Christian University, he was given the nickname "The Mississippi Gambler". Day completed 111 of 233 passes for 2,022 yards and 14 touchdowns during his Ole Miss career. Not counting bowl statistics, he had 2,428 yards of total offense and was responsible for 21 touchdowns. His first pass in college (vs. Chattanooga) in 1953 was a 63-yard touchdown.
Although Day was drafted by the Washington Redskins in the 17th round of the 1956 NFL Draft, he did not join the Redskins until the 1959 and 1960 seasons, instead joining the Winnipeg Blue Bombers first for three years first.
In 1961, Day left the NFL for the CFL, joining the Calgary Stampeders, with which team he remained through the 1965 season. In 1962, he was an all-star and won the Jeff Nicklin Memorial Trophy. When he left the Stampeders, he joined the Toronto Argonauts (the first of two Ole Miss stars to play quarterback for Toronto, the other being Kent Austin in 1995).
At the time of his death, Day's 226 playoff competitions placed him fifth all-time, while his 3,132 playoff passing yards had him sixth for all-time.