Why in the world is there even an April Fool's Day? I asked myself this question, and of course I had to find out.
There are various different references and theories, but the one I came across the most indicated that it most likely came to be at the time when parts of Europe changed from using the old Julian calendar to the new Gregorian calendar. Using the Julian calendar, the new year was celebrated around April 1. The Gregorian calendar put the new year at January 1. So the people who weren't completely up to speed still wanted to celebrate the new year on April 1st. So they were called fools, mocked and ridiculed for either not knowing or because they didn't want to change to the new calendar.
To humiliate people on April Fool's, the French would stick fish to the backs of people who still celebrated on April 1st. Gotta love the French!