John's brothers included William Day, who trained three classic winners, and the successful jockeys Samuel and Alfred.

Day's successes for the Duke included the 2000 Guineas winner Vauban and the classic winning fillies Siberia and Scottish Queen.

All three of these horses were ridden by George Fordham who was the stable jockey at Danebury for several seasons, taking over the role previously occupied by the trainer's brother Alfred.

[2] Day was accused of knowing that the filly had lost her form, and of misleading Hastings and others as to her condition while wagering heavily against her.

When Henry John Rous made allegations about Day's behaviour in a letter to The Times, the trainer responded by bringing an action for libel.