Battle of the Boyne: Why is it in the iPhone calendar and what is it?

The Battle of the Boyne has arrived – a moment greeted with incredible significance by some people in the UK, and confusion by others. The event is a public holiday in Northern Ireland and marks one of the most controversial and important parts of British history. And it is marked by a message in people's iPhone calendar, which has the tendency to confuse some people who see it. For people in Northern Ireland, that fact is obvious: they have the day off, and the controversial celebrations are hard to miss. But for the rest of the UK, the only reminder might be a cryptic note inside their iPhone calendar.