Did I fool you? No? I didn’t really think I would. But did you ever wonder why April 1st is celebrated as April Fools’ Day?

According to one theory, it could actually have a lot to do with the New Year and the Pope. Pope Gregory XIII, that is. You know, the guy that introduced the Gregorian Calendar? The calendar the majority of the world uses today.

In 1582, Pope Gregory XIII introduced a new standard calendar for Christian Europe that would take his name and centuries later become the standard internationally in the 21st century.

Prior to the 15th century, Europe’s nations and city states operated using the Julian calendar. The Gregorian calendar moved the date of the new year from April 1 to January 1, among other changes. Catholic monarchies were naturally its earliest adopters, though Protestant nations later followed suit.

Given the nature of the reform, both in terms of communicating such a fundamental change to a large population and dealing with critics of the new calendar, some Europeans continued to celebrate the new year between March 25 and April 1. April fools were those who still celebrated the holiday in the spring, and were the subject of pranks and ridicule by those who observed the new year months ago.

It is worth noting that many different cultures have had days of foolishness around the start of April, give or take a couple of weeks. The Romans had a festival named Hilaria, the Hindu calendar has Holi, and the Jewish calendar has Purim. Perhaps there’s something about the time of year, with its turn from winter to spring, that lends itself to lighthearted celebrations.