Just found this information about April Fools Day
The first of April, some do say,
Is set apart for All Fools' Day.
But why the people call it so,
Nor I, nor they themselves do know.
But on this day are people sent
On purpose for pure merriment.
Poor Robin's Almanac (1790)
The first of April is the day we remember what we are the other 364 days of the year. -
Mark Twain

The custom of this day is to play jokes or send friends on fools' errands. Its timing seems to be related to spring, when nature frequently "fools" mankind with changes in weather. This custom was brought to America by the British, however, many believe that April Fools' Day began in France. March 25th used to be New Year's Day and April 1st was when the festivities culminated and ended. Pope Gregory introduced a new calendar for the Christian world in 1562. The new year fell on January first. Many forgot and some did not believe the change in the date and continued to celebrate New Year's Day on the old date. Others played tricks on them and dubbed them "April fools" sending them on a "fool's errand" or attempted to have them believe that something false was true.

In France, April 1st is called "Poisson d'Avril." French children fool their friends by taping a paper fish to their friends' backs. When the "young fool" discovers this trick, the prankster yells "Poisson d’Avril!" (April Fish!) In England, April Fools' tricks are played in the morning and if a trick is played on you, you are called a "noddie". In Scotland, you are called a "gowk" (cuckoo bird which symbolizes or represents a simpleton) with a favorite prank being a "Kick Me" sign. In Portugal, you celebrate twice (the Sunday and Monday before Lent) by throwing flour at your friends. In colonial America one might have been asked to get a copy of "The History of Adam's Grandfather" or obtain some "sweet vinegar." One common trick on April Fool's Day, or All Fool's Day, is pointing down to a friend's shoe and saying, "Your shoelace is untied." Teachers in the 19th century would exclaim, "Look! A flock of geese!" and point up. School children might tell a classmate that school has been cancelled. Boy scouts or their leaders will send an innocent to look for a "smoke sifter". Sometimes, college students would set their roommate's clock an hour behind so they would show up in the wrong class or not at all. Secretaries have frequently left "urgent" messages for their employer that an important telephone call from Mr. Lyon came in with the return telephone number to the local zoo. Whatever the trick, if the innocent victim falls for the joke the prankster yells, "April Fool!" Most April Fool jokes are good, clean fun and not meant to harm anyone. The idea is for everyone to laugh, especially the "innocent" victim.

I found this quite interesting.