Day is the son of Xcel Motorsport team director Kevin Day and was born in the United Kingdom, though he has lived in Dubai for most of his life. He began karting at the age of six, achieving notable success with a win in the 2017 MSA Kartmasters GP in Mini Max and the 2019 FFSA Karting OK-J title. Day was also a factory driver for Leclerc by Lennox Racing, earning race wins in the Champions of the Future, Karting European Championship and WSK Euro Series.
Day transitioned to single-seaters in 2021, joining Xcel Motorsport in the Formula 4 UAE Championship. He secured his first win at Yas Marina, and finished third in race two at Dubai, ultimately finishing sixth in the championship standings. He returned to Xcel Motorsport for a second season in 2022, taking a podium at Yas Marina but finishing 13th overall.
In 2022, Day switched to GT4 racing, joining R Racing in the British GT Championship alongside Josh Miller. The pair became the youngest winning pairing in the series, securing class wins at Snetterton and Spa to finish third in the GT4 standings. He made a one-off appearance for Mirage Racing in the GT4 European Series at Barcelona at the end of the year.
Day continued in GT4 competition with Mirage Racing in 2023, competing in a full season of the GT4 European Series. Racing in the Silver class alongside Ruben del Sarte, the duo achieved a single class podium at Spa and finished 15th in points. He then joined Forsetti Motorsport for 2024, winning the GT4 Winter Series before racing in the GT4 class of the British GT Championship. Day won the season-opening race at Oulton Park and scored four more podiums to end the season as the runner-up in the GT4 overall standings, also claiming the Silver cup championship. During 2024, he also competed for Racing Spirit of Léman in the GT4 European Series, scoring three podiums in the final four races to finish fifth in the Silver cup standings.
Day made his GT3 debut with PROsport Racing in ADAC GT Masters in 2024, and joined Earl Bamber Motorsport for the final two rounds of the 2024–25 Asian Le Mans Series. He secured his first GT3 podium in race two at Dubai, and followed it up with a second-place finish at Yas Marina to finish fifth in points.
For the remainder of 2025, Day joined Comtoyou Racing for a dual programme in both the GT World Challenge Europe Endurance and Sprint Cups after graduating from the AMR Driver Academy. He finished 33rd in the Silver Cup standings of the Endurance Cup, but found success in the Sprint Cup, scoring three class wins and two more podiums to secure the Silver Cup title at the season finale in Valencia alongside Kobe Pauwels. In 2025, Day also competed in GT World Challenge Australia for Volante Rosso Motorsport alongside Liam Talbot, winning on his debut round at Sandown. He returned to EBM for the first round of the 2025–26 Asian Le Mans Series at Sepang, achieving a best result of seventh.
In 2026, Day joined Walkenhorst Motorsport for a dual campaign in the GT World Challenge Europe Endurance and Sprint Cups, and returned to Comtoyou for his first full season in ADAC GT Masters.