It's one of the world's most widely observed traditions — and nobody can explain how it started.
The leading theory points to France in 1564, when King Charles IX moved New Year's Day from April 1st to January 1st. People who kept celebrating on the old date got mocked as "April fools." Sounds neat, except historians can't actually prove the connection.
Other candidates: the ancient Roman festival Hilaria (end of March, people wore disguises and pranked each other), and a possible reference in Chaucer's Canterbury Tales from 1392 — though scholars now think that's a medieval typo.
The first confirmed British prank was in 1698 — Londoners were tricked into visiting the Tower of London to "see the lions washed." There were no lions being washed.
In France, it's called Poisson d'Avril — "April Fish." Children stick paper fish on each other's backs. In Scotland, it's a two-day event: day one is "Hunt the Gowk" (send someone on a fake errand), day two is "Tailie Day" (pin "kick me" signs on people).
The corporate era brought bigger hoaxes. In 1957, the BBC reported that Swiss farmers were harvesting spaghetti from trees — viewers called in asking how to grow their own. In 1996, Taco Bell announced it had purchased the Liberty Bell. In 1998, Burger King advertised a "Left-Handed Whopper." People ordered it.
After 500+ years, the only thing anyone agrees on: don't believe anything you read on April 1st.