Day was born in Melbourne, Victoria. His father, Alfred Day, was an engineer. Referring to his early years, Day was quoted as:^[1]

I grew up in the metropolitan area of Melbourne during the depression. My father deserted us when we were kids, so we had a fairly rough upbringing; my mother had to bring up three of us without the benefit of any deserted wives' pension. We scrabbled very hard and that sort of builds a fire in your guts. It leaves a mark on you really that never leaves you. The scars are there from charity handouts and all those rather soul-destroying experiences. That's why I'll always be Labor.

He was educated at Swinburne Technical College and trained as a fitter and turner. In 1941 he joined the Australian Imperial Forces as a private and later transferred to the Royal Australian Air Force to become a catalina pilot where he met and married Marie Davis.^[2]

Following his war service Day and his wife settled in her home town of Maclean and established a car and farm equipment dealership. He joined the Labor party at age 28 and served in local government.^[3]

Day won Labor pre-selection and was subsequently elected as the member for Casino at the 1971 state election; the first time the seat had been held by Labor since the establishment of single member constituencies in 1927.^[2] He retained this seat until it was abolished in a redistribution prior to the 1981 state election. He subsequently contested and won the seat of Clarence. With the election of the Wran government in 1976, Day, one of the few rural based elected members of the Labor Party, became the Minister for Decentralisation and Development and Minister for Primary Industries. He continued in rural industry portfolios in the Wran government until his retirement prior to the 1984 state election.^[3]