You see, the Battle of the Boyne was fought on the 1st July 1690 under the Julian calendar. In 1750, when Britain adopted the Gregorian calendar, ten days were added to bring Britain in line with Europe, which makes the 1st July Old Style into 11th July New Style. The bonfires on the evening of the 11th are as if the word is just reaching the towns and villages, and then the celebrations and processions are held on the next day, the Twelfth, unless it's on a Sunday.
The only thing that happened on 12th July (OS) was the Battle of Aughrim in 1691, the truly decisive battle of the Williamite Wars in Ireland. King Billy was long gone, though, safely in London governing his kingdoms. But then the actual anniversary of Aughrim would be the 22nd July according to our calendars.
So the glorious Twelfth is here. But how many Orangemen will realise that it's all down to Pope Gregory that they're marching on the Twelfth?