Playing for then-offensive coordinator Chip Kelly, Day set four career records at UNH including completion percentage and touchdowns.

[10] On January 3, 2017, Day was hired to replace co-offensive coordinator Tim Beck of the Ohio State Buckeyes.

[12] On August 1, 2018, Day, was named acting head coach at Ohio State when head coach Urban Meyer was placed on administrative leave[13] when news came to light of Meyer's knowledge of events surrounding then-fired Zach Smith's domestic violence accusations made by Smith's estranged wife.

[14] On December 4, 2018, Ohio State announced that Meyer would retire as head coach after the 2019 Rose Bowl and be replaced by Day on a full-time basis.

[20] The Buckeyes were named the number two seed in the College Football Playoff and lost to the Clemson Tigers in the Fiesta Bowl.

[23] Day's second season was significantly shortened due to the Big Ten Conference's policies regarding the COVID-19 pandemic.

[25][26][27] After starting the season 4–0, Coach Day was forced to miss the December 5 game against Michigan State after testing positive for COVID-19,[28] which the Buckeyes won, 52–12.

[29] However, on December 9, 2020, the Big Ten administrative council voted to remove the six game minimum, allowing Ohio State to advance to the conference championship.

[30] The Buckeyes played in the Big Ten Championship game on December 19, where they beat the Northwestern Wildcats 22–10.

[32] Ohio State defeated Clemson 49–28, and advanced to the 2021 College Football Playoff National Championship, to face number 1 Alabama.

[35][36] After an early season upset by Oregon, Day led Ohio State through a nine-game win streak behind Heisman finalist C. J.

[43][44] In the inaugural College Football Playoff rankings of the season, the Buckeyes landed at #2, a spot they would hold for four weeks.

[53] In the College Football Playoff Semifinal at the Chick-fil-A Peach Bowl, Day and Buckeyes fell to the eventual National Champion Georgia Bulldogs 42–41.

[66] After a close win against Nebraska at home, they beat #3 Penn State, and won against #5 Indiana leading into the Michigan game.

The Buckeyes entered the Michigan game ranked #2 for the fourth straight year, and as 23.5 point favorites, the largest in history.

[71] With the newly implemented expanded playoff, they were set to battle Tennessee as the eight-seed at home in the first round.

Although there was much talk about Tennessee and their fans taking over Ohio Stadium, Day and the Buckeyes quickly put that to rest with touchdowns on the first three drives en route to a 42–17 victory, setting up a rematch against one-seed Oregon in the historic Rose Bowl.

[73] The highly anticipated matchup was not the battle many thought it would be, as the Buckeyes once again came out swinging, holding a 34–0 lead before a last-second touchdown before the half made it 34–7 going into halftime.

[76] Day met his wife, Christina Ourania Spirou, whom he calls Nina, when they played together on the same tee-ball team when she was seven and he was six.

Since becoming head coach at Ohio State, Day and his wife have chosen to partner with an organization focused on removing stigmas related to mental health called On Our Sleeves, a movement begun at the Nationwide Children's Hospital.