Around the world, April 1 is a day when people try to fool their friends. In France, April Fool's Day is called "Poisson d'avril." In English that means "fish of April."
A popular joke, especially with schoolchildren, is to hook or stick a cardboard fish onto someone's back. When the joke is discovered, the trickster shouts, "Poisson d'avril" (April Fish!)
What does a fish have to do with April Fool's Day?
Some people think that it is because there were always lots of fish in French streams and rivers during early April after the baby fish had hatched. These young fish were easy to fool with a hook and lure, and were very easy to catch. The French called them "Poisson d'avril" or "April Fish." It then became a custom to fool people on April 1, as a way of celebrating all foolish fish.
Others say the fish custom is because the zodiac sign of Pisces, occurring just before April, is a fish.
On "April Fish Day" in France, bakeries are full of cookies and cakes shaped like fish, and people often give each other chocolate fish. 
Source: worksheetsplus.com/Reading/AprilFish.html April Fish!
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