I know it was yesterday, but I had the "Furkid Friday" post to do yesterday, so you can have this a day late, since I want to post it after I went to the trouble of finding the information!
"April Fools' Day is celebrated in the Western world on April 1st of every year. Sometimes referred to as All Fools' Day, April 1st is not a legal holiday, but is widely recognized and celebrated as a day which tolerates practical jokes and general foolishness. The day is marked by the commission of good humoured or funny jokes, hoaxes and other practical jokes of varying sophistication on friends, family members, teachers, neighbors, work associates, etc.
Traditionally, in some countries such as New Zealand, the UK, Australia, and South Africa, the jokes only last until noon, and someone who plays a trick after noon is called an "April Fool". It is for this reason that newspapers in the U.K. that run a front page April fool only do so on the first (morning) edition. Elsewhere, such as in France, Ireland, Italy, South Korea, Japan, Russia, The Netherlands, Germany, Brazil, Canada, and the U.S., the jokes last all day. 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.
In Chaucer's Canterbury Tales (1392), the "Nun's Priest's Tale" is set Syn March bigan thritty dayes and two. Chaucer probably meant 32 days after March, i.e. May 2, the anniversary of the engagement of King Richard II of England to Anne of Bohemia, which took place in 1381. However, readers apparently misunderstood this line to mean "March 32," i.e. April 1. In Chaucer's tale, the vain cock Chauntecleer is tricked by a fox.
In 1508, a French poet referred to a poisson d’avril (April fool, literally "April fish"), a possible reference to the holiday. In 1539, Flemish poet Eduard de Dene wrote of a nobleman who sent his servants on foolish errands on April 1. In 1686, John Aubrey referred to the holiday as "Fooles holy day", the first British reference. On April 1, 1698, several people were tricked into going to the Tower of London to "see the Lions washed". The name "April Fools" echoes that of the Feast of Fools, a Medieval holiday held on December 28.
In the Middle Ages, New Year's Day was celebrated on March 25 in most European towns. In some areas of France, New Year's was a week-long holiday ending on April 1. So it is possible that April Fools originated because those who celebrated on January 1 made fun of those who celebrated on other dates. The use of January 1 as New Year's Day was common in France by the mid-sixteenth century, and this date was adopted officially in 1564 by the Edict of Roussillon.
In the eighteenth century the festival was often posited as going back to the time of Noah. According to an English newspaper article published in 1789, the day had its origin when Noah sent his dove off too early, before the waters had receded; he did this on the first day of the Hebrew month that corresponds with April."
(Above taken from this page where you can read more on April Fools Day, if you like).