Also known as The Twelfth or Orangemen's Day, the Battle of the Boyne is an annual public holiday in Northern Ireland observed on July 12, or the Monday after if July 12 falls on a weekend. It celebrates the battle fought by King William of Orange against King James II in 1690, a key moment in the "Glorious Revolution" when the Protestant William overthrew the Catholic James with the support of the English Parliamentarians.

The holiday is celebrated with parades organized by the Orange Lodge of Ireland in towns across Northern Ireland. On the night of July 11, bonfires are lit to commemorate fires lit to to help King William navigate a landing at Carrickfergus Castle.

In addition to the Bank Holiday, some Northern Ireland businesses close for up to two weeks in a shutdown known as the Twelfth Fortnight.