"to mark the Glorious Twelfth"

The Grouse Shoot
Public DomainThe Grouse Shoot - Credit: Heywood Hardy
The Glorious Twelfth is 12 August, the start of the grouse shooting season in Britain.

A different 12th is celebrated in Northern Ireland by the Protestant Orange Order.  On 12 July 1690 William of Orange, a Dutch Protestant King, defeated the Catholic King James II at the Battle of the Boyne. The Orange Order march through the streets of Northern Ireland every 12 July, to the chagrin of their Catholic neighbours.