The PE teacher at Pembroke School, Neil Truman, recommended Day for the Pembrokeshire 'Elite' squad, and at the age of 18, he was invited to join the Scarlets' academy.
Day made his regional debut for the Scarlets against Northampton Saints on 11 August 2006 in a pre-season friendly ahead of the 2006–07 season.
That year, he became a regular in the second row for Llanelli, and scored his first senior try in another friendly against Pembroke Dock Quins RFC on 16 August.
After featuring in both pre-season friendlies against Exeter Chiefs and Bath, Day was named in the starting line-up for the league season opener at home to the Newport Gwent Dragons; however, he was substituted at half-time as the Scarlets lost 30–23 and he did not play again for the region until January 2008, coming on for Adam Eustace in the 66th minute of a 40–7 away defeat to London Wasps in the Heineken Cup.
However, with the announcement of the introduction of a salary cap into Welsh regional rugby for 2012–13, the Scarlets elected not to extend Day's contract, and in February 2012, he signed a three-year deal to move to English team Bath at the end of the 2011–12 season.
[8] His time at the Scarlets ended on a high note as he scored the team's fourth try in a 26–14 away win over the Cardiff Blues on 24 March 2012.
[citation needed] After nearly three seasons with Saracens, a recurring knee injury forced Day to retire from professional rugby in November 2019.