Via Big Think: ‘Many conflicting theories exist that try and pinpoint the origins of the holiday everyone in your office hates you for. Of all these theories, the most likely root of what we now know as April Fools’ Day dates back to Pope Gregory XIII, who reigned — or if reign isn’t the right word — who pope’d from 1572 to 1584. I’m sure you’re familiar with the calendar hanging on your wall that starts in January, ends in December, and consists of seemingly arbitrary amounts of days per month. You can thank Pope Greg for that. His Gregorian calendar replaced the Julian Calendar in 1582.

One major change with the calendar switch was that New Year’s Day moved from the end of March to the beginning of January. As Tech Times notes, those who didn’t get the memo about the change of date and celebrated the old New Year’s Day at the end of March were thus deemed, naturally, April fools.