A member of a highly successful racing family, Day first made his name as a jockey in the 1820s and rode the winners of sixteen classics before retiring.

Horses trained by Day won seven classics between 1838 and 1854, during which time he was regarded as the leading trainer in the South of England and the main rival of the Yorkshire-based John Scott.

[3] Day had further classic success for his patron, winning The Oaks on Turquoise in 1828 and Oxygen in 1831, and also rode several winners for King George IV.

His first major patron was the wealthy and powerful Lord George Bentinck who invested heavily in the Danebury yard, enabling Day to build one of the largest and best-equipped training establishments in the country.

Samuel Goddard Day (c.1818–1838), not to be confused with his uncle, was a talented jockey who won the St Leger on Mango in 1837 but died after a hunting accident eight months later at the age of 19.

As a trainer he took over the Danebury stable and eclipsed his father's success by winning twelve classics including three Derbys with Pyrrhus the First, Cossack and Andover.

[1] William Day (1823–1908) trained at Woodyates and was responsible for sending out Lord of the Isles to beat St Hubert in the 1855 2000 Guineas, the race which effectively ended his father's career.

Whilst training at Michel Grove, Day took on an apprentice named John Porter who went on to become one of the most successful British trainers of the late 19th century.