Day played rugby union at the age of eight[citation needed], and at the age of 15 starting as a Number 8before being moved to the second row. 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.[3] Around the same time, he applied to the University of Wales Institute, Cardiff (UWIC) to study for a degree in sports science. In his first year there, he played for the university rugby team, then Scarlets feeder club Carmarthen Quins RFC in his second year, before joining Llanelli RFC for the 2005–06 season in his final year.[3] He made his debut on 21 January 2006. At international level, Day played has played for Wales at under-19, under-21 and Senior levels. In 2004, he played in the U19 World Championships in South Africa, and earned 10 caps overall for the under-19s. The following year, he also participated in the U21 World Championships in Argentina before being selected for the same tournament in France in 2006. In total, he earned 15 caps for the under-21s.[10] In June 2015, Day was named in Wales’ initial training squad for the World Cup later in the year.[11] Day made his full international debut aged 29 in the starting line-up for Wales against Ireland on 8 August 2015. In January 2016, Day was named in the Wales squad for the Six Nations Championship.