April Fools Day

April Fools' Day is celebrated in different countries around the world on April 1 every year. Sometimes referred to as All Fools' Day, April 1 is not a national holiday, but is widely recognized and celebrated as a day when many people play all kinds of jokes and foolishness. The day is marked by the commission of good-humoured or otherwise funny jokes, hoaxes, and other practical jokes of varying sophistication on friends, family members, teachers, neighbors, work associates, etc.
The earliest recorded association between April 1 and foolishness can be found in Chaucer's Canterbury Tales (1392). Many writers suggest that the restoration of January 1 as New Year's Day in the 16th century was responsible for the creation of the holiday, but this theory does not explain earlier references.

Precursors of April Fools Day include the Roman festival of Hilaria, held March 25,^[1] and the Medieval Festival of Fools, held December 28,^[2] still a day on which pranks are played in Spanish-speaking countries.
In Chaucer's Canterbury Tales (1392), the "Nun's Priest's Tale" is set Syn March bigan thritty dayes and two.^[3] Modern scholars believe that there is a copying error in the extant manuscripts and that Chaucer actually wrote, Syn March was gon.^[4] Thus the passage originally meant 32 days after March, i.e. May 2,^[5] the anniversary of the engagement of King Richard II of England to Anne of Bohemia, which took place in 1381. Readers apparently misunderstood this line to mean "March 32", i.e. April 1.^[6] In Chaucer's tale, the vain cock Chauntecleer is tricked by a fox.
In 1508, French poet Eloy d'Amerval referred to a poisson d’avril (April fool, literally "April fish"), a possible reference to the holiday.^[7] In 1539, Flemish poet Eduard de Dene wrote of a nobleman who sent his servants on foolish errands on April 1.^[5] In 1686, John Aubrey referred to the holiday as "Fooles holy day", the first British reference.^[5] On April 1, 1698, several people were tricked into going to the Tower of London to "see the Lions washed".^[5]
In the Middle Ages, New Year's Day was celebrated on March 25 in most European towns.^[8] In some areas of France, New Year's was a week-long holiday ending on April 1.^[2][1] Many writers suggest that April Fools originated because those who celebrated on the January 1 made fun of those who celebrated on other dates.^[1] The use of the January 1 as New Year's Day was common in France by the mid-sixteenth century,^[5] and this date was adopted officially in 1564 by the Edict of Roussillon.

from http://en.wikipedia.org/wiki/April_Fools%27_Day