The origins of April Fools are complex and a matter of debate. It is likely a relic of the once common festivities held on the vernal equinox, which began on the 25th of March, old New Year's Day, and ended on the 2nd of April.
Though the first of April appears to have been observed as a general festival in ancient Britain, it was apparently not until the beginning of the 18th century that the making of April Fools was a common custom. In Scotland the custom was known as "hunting the gowk," i.e. the cuckoo, and April Fools were "April-gowks," the cuckoo being a term of contempt, as it is in many countries.
One of the earliest connections of the day with fools is Chaucer's story the Nun's Priest's Tale (c.1400), which concerns two fools and takes place "thritty dayes and two" from the beginning of March, which is April 1. The significance of this is difficult to determine.
Europe may have derived its April-fooling from the French. French and Dutch references from 1508 and 1539 respectively describe April Fools Day jokes and the custom of making them on the first of April. France was one of the first nations to make January 1 officially New Year's Day (which was already celebrated by many), by decree of Charles IX. This was in 1564, even before the 1582 adoption of the Gregorian calendar.
Thus the New Year's gifts and visits which had been the feature of the first of April became associated with the first day of January, and those who disliked or did not hear about the change were fair game for those wits who amused themselves by sending mock presents and paying calls of pretended ceremony on the first of April.
In France the person fooled is known as poisson d'avril (April fish). This has been explained as arising from the fact that in April the sun quits the zodiacal sign of the fish. The French traditionally celebrated this holiday by placing dead fish on the backs of friends. Today, real fish have been replaced with sticky, fish-shaped paper cut-outs that children try to sneak onto the back of friends' shirts. Candy shops and bakeries also offer fish-shaped sweets for the holiday.
Alabama Changes the Value of Pi: The April 1998 newsletter of New Mexicans for Science and Reason contained an article written by physicist Mark Boslough claiming that the Alabama Legislature had voted to change the value of the mathematical constant pi to the "Biblical value" of 3.0. This claim originally appeared as a news story in the 1961 science fiction novel Stranger in a Strange Land by Robert A. Heinlein.
Spaghetti trees: The BBC television program Panorama ran a famous hoax in 1957, showing the Swiss harvesting spaghetti from trees. They had claimed that the despised pest the spaghetti weevil had been eradicated. A large number of people contacted the BBC wanting to know how to cultivate their own spaghetti trees.
Left Handed Whoppers: In 1998, Burger King ran an ad in USA Today, saying that people could get a Whopper for left-handed people whose condiments were designed to drip out of the right side. Not only did customers order the new burgers, but some specifically requested the "old", right-handed burger.
Taco Liberty Bell: In 1996, Taco Bell took out a full-page advertisement in The New York Times announcing that they had purchased the Liberty Bell to "reduce the country's debt" and renamed it the "Taco Liberty Bell." When asked about the sale, White House press secretary Mike McCurry replied tongue-in-cheek that the Lincoln Memorial had also been sold and would henceforth be known as the Ford Lincoln Mercury Memorial.
San Serriffe: The Guardian newspaper in England printed a supplement in 1977 praising this fictional resort, its two main islands (Upper Caisse and Lower Caisse), its capital (Bodoni), and its leader (General Pica). Intrigued readers were later disappointed to learn that San Serriffe (sans serif) did not exist except as references to typeface terminology. (This comes from a Jorge Luis Borges story.)
Metric time: Repeated several times in various countries, this hoax involves claiming that the time system will be changed to one in which units of time are based on powers of 10.
Smell-o-vision: In 1965, the BBC purported to conduct a trial of a new technology allowing the transmission of odor over the airwaves to all viewers. Many viewers reportedly contacted the BBC to report the trial's success.
Tower of Pisa: The Dutch television news reported once in the 1950s that the Tower of Pisa had fallen over. Many shocked people contacted the station.
Write Only Memory: Signetics advertised Write Only Memory IC databooks in 1972 through the late 1970s.
Annual BMW Innovations see a new "cutting-edge invention" by BMW advertised across British newspapers every year, examples including: Warning against counterfeit BMWs: the blue and white parts of the logo were reversed. The "Toot and Calm Horn" (after Tutankhamun), which calms rather than aggravates other drivers, so reducing the risk of road rage. MINI cars being used in upcoming space missions to Mars. IDS ("Insect Deflector Screen") Technology - using elastic solutions to bounce insects off the windscreen as you drive. SHEF ("Satellite Hypersensitive Electromagnetic Foodration") Technology, which sees the car's GPS systems synchronise with home appliances to perfectly cook a meal for the instant you return home.
Sheng Long - Electronic Gaming Monthly's infamous hoax of a secret character in Street Fighter II.
There have been several other EGM pranks that readers have fallen for. Among them: claiming that some Street Fighter II characters possessed unlisted special moves, including Chun-Li hurling her bracelets at an opponent, Sega mascots Sonic and Tails appearing as playable characters in Super Smash Bros. Melee, and the release of a graphically-remade The Legend of Zelda: The Wind Waker as a preorder bonus. All such pranks have been met with praise and equal hatred from its readers, as can be seen in the "April Fools" letters section in the May issue.
EGM repeated the Sheng Long hoax again with Street Fighter III. (info from Google)
By the way, I made up the 1703 date for the first April Fools Day up at the top. April Fools! I also confess to a decade-long series of reports of fake news about the phone business. CLICK for the latest edition.