From History.com with my emphases and comments:
On this day in 1700, English pranksters begin popularizing the annual tradition of April Fools’ Day by playing practical jokes on each other.
Although the day, also called All Fools’ Day, has been celebrated for several centuries by different cultures, its exact origins remain a mystery. 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. 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. These 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. [In Italy we still say "Pesci! Fish!" on this say when some prank is revealed.]
Historians have also linked April Fools’ Day to ancient festivals such as Hilaria, which was celebrated in Rome at the end of March and involved people dressing up in disguises. 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. [Yah... right.]
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) [I use this in one of my blog tags: Puir Slow-Witted Gowk. I picked it up from a character in Patrick O'Brian's great books.] 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 Web sites 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; numerous viewers were fooled. [Ah yes! But little do people realize that that's true!] In 1985, Sports Illustrated tricked many of its readers when it ran a made-up article about a rookie pitcher named Sidd Finch who could throw a fastball over 168 miles per hour. [Best joke ehvurrr.] 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. [BTW, that the left-handed version wasn't nearly as good.]