William's brothers included John Day, who trained twelve classic winners, and the successful jockeys Samuel and Alfred.
When still in his early twenties, Day was involved in a scandal surrounding a horse named Old England, trained by his father and owned by John Gully.
Old England was fancied for the 1845 Epsom Derby, but Day wagered heavily against the colt and was involved in a plot to "nobble" (deliberately injure) it at the Danebury stable.
In 1852, he trained his first major winner when he sent out Joe Miller to defeat forty-two rivals the Chester Cup, at that time one of the year's most important and valuable races.
According to one version of events,[3] William and John Sr. came to an arrangement whereby Lord of the Isles would be allowed to win the 2000 Guineas, but would not be trained seriously for the Derby, while the reverse would apply to St Hubert.
When St Hubert was withdrawn from the Derby, the Day family were forced to rely on Kingstown and Lord of the Isles, despite the fact that the latter had hardly been trained since the Guineas.
Both William and John Jr. were strongly suspected of involvement in a series of plots to "nobble" the other leading fancy Wild Dayrell, who nevertheless won the race, with Kingstown second and Lord of the Isles third.
Four years later, Day won a second classic when he trained his own horse, The Promised Land, to win the 2000 Guineas, ridden by his younger brother Alfred.
He left Woodyates to train at his own Cholderton Lodge Estate near Grateley, Hampshire, but retired shortly afterwards and lived at Salisbury till his death in 1908.