Constituting number twelve in a sequence. The phrase The Twelfth denotes both 12 July, celebrated by upholders of Protestant supremacy in Ireland as the anniversary of William III's victory over James II at the Battle of the Boyne, and (in the UK) 12 August, the day on which the grouse-shooting season begins.
twelfth man in cricket, a player nominated to act as a reserve in a game, typically carrying out duties such as fielding as a substitute and taking out drinks.
Twelfth Night 6 January, the feast of the Epiphany. Strictly, this denotes the evening of 5 January, the eve of the Epiphany and formerly the twelfth and last day of Christmas festivities.