Happy Day, Fool!

From Wikipedia - the most reliable on-line source, hands down:

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The origin of April Fools' Day is obscure. One likely theory is that the modern holiday was first celebrated soon after the adoption of the Gregorian Calendar; the term referred to someone still adhering to the Julian Calendar, which it replaced.[2] In many pre-Christian cultures May Day (May 1) was celebrated as the first day of summer, and signalled the start of the spring planting season. An April Fool was someone who did this prematurely. Another origin is that April 1 was counted the first day of the year in France. When King Charles IX changed that to January 1, some people stayed with April 1. Those who did were called "April Fools" and were taunted by their neighbors.[citation needed] In the eighteenth century the festival was often posited as going back to the times of Noah. An English newspaper article published on April 13th, 1789 said that the day had its origins when he sent the raven off too early, before the waters had receded. He did this on the first day of the Hebrew month that corresponds with April.[3] A possible reference to April Fools' Day can be seen in the Canterbury Tales (ca 1400) in the Nun's Priest's tale, a tale of two fools: Chanticleer and the fox, which took place on March 32nd.

Well-Known Pranks:

• 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 programme 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. It was, in fact, filmed in St Albans.
  
• 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 Lincoln Mercury Memorial.
  
• San Serriffe: The Guardian 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.  In 2007, the BBC website repeated an online version of the hoax.
  
• 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.
  
• The Canadian news site bourque.org announced in 2002 that Finance Minister Paul Martin had resigned "in order to breed prize Charolais cattle and handsome Fawn Runner ducks."