Some historians speculate that April Fools' Day dates back to 1582, when France switched from the Julian calendar to the Gregorian calendar, as called for by the Council of Trent in 1563. In the Julian Calendar, as in the Hindu calendar, the new year began with the spring equinox around April 1.

People who were slow to get the news or failed to recognize that the start of the new year had moved to January 1 and continued to celebrate it during the last week of March through April 1 became the butt of jokes and hoaxes and were called “April fools.” These pranks included having paper fish placed on their backs and being referred to as “poisson d’avril” (April fish), said to symbolize a young, easily caught fish and a gullible person.

Historians have also linked April Fools' Day to festivals such as Hilaria (Latin for joyful), which was celebrated in ancient Rome at the end of March by followers of the cult of Cybele. It involved people dressing up in disguises and mocking fellow citizens and even magistrates and was said to be inspired by the Egyptian legend of Isis, Osiris and Seth.

There’s also speculation that April Fools’ Day was tied to the vernal equinox, or first day of spring in the Northern Hemisphere, when Mother Nature fooled people with changing, unpredictable weather.

April Fools' Day spread throughout Britain during the 18th century. In Scotland, the tradition became a two-day event, starting with “hunting the gowk,” in which people were sent on phony errands (gowk is a word for cuckoo bird, a symbol for fool) and followed by Tailie Day, which involved pranks played on people's derrieres, such as pinning fake tails or “kick me” signs on them.

In modern times, people have gone to great lengths to create elaborate April Fools' Day hoaxes. Newspapers, radio and TV stations and websites have participated in the April 1 tradition of reporting outrageous fictional claims that have fooled their audiences.

In 1957, the BBC reported that Swiss farmers were experiencing a record spaghetti crop and showed footage of people harvesting noodles from trees. In 1985, Sports Illustrated writer George Plimpton tricked many readers when he ran a made-up article about a rookie pitcher named Sidd Finch who could throw a fastball over 168 miles per hour.

In 1992, National Public Radio ran a spot with former President Richard Nixon saying he was running for president again… only it was an actor, not Nixon, and the segment was all an April Fools' Day prank that caught the country by surprise.

In 1996, Taco Bell, the fast-food restaurant chain, duped people when it announced it had agreed to purchase Philadelphia's Liberty Bell and intended to rename it the Taco Liberty Bell. In 1998, after Burger King advertised a “Left-Handed Whopper,” scores of clueless customers requested the fake sandwich. Google notoriously hosts an annual April Fools' Day prank that has included everything from “telepathic search” to the ability to play Pac Man on Google Maps.

For the average trickster, there is always the classic April Fools' Day prank of covering the toilet with plastic wrap or swapping the contents of sugar and salt containers.

A Brief, Totally Sincere History of April Fools’ Day. Washington Post.
History’s Greatest April Fools Jokes. National Geographic.
Some of the greatest April Fools' pranks of all time. CNN.
15 Best April Fools’ Day Hoaxes. CBS.