Poisson d'Avril (April Fish) is the French equivalent for "April Fool's Day." On this day, the joke is on you if you (or someone else) finds that there is a fish taped to your back.
Here's something about the origin of Poisson d'Avril (David, feel free to correct me if I've got something wrong).
In France, it was said that until 1564, the year began on 1 April. That year, King Charles IX decided to alter the timetable. The year now begins on 1 January.On 1 January 1565 everyone wished- "Happy New Year" to friends and family, gave gifts, etc. Only when Poisson d'Avril came on April 1st, some petits farceurs ("little jokers") had the idea to do continue to give gifts, as was previously the tradition. But, as they were jokers and it was no longer the "real" beginning of the year, their gifts were false gifts, gifts "to make you laugh," simple as that!
From that day on, every year on 1 April, everyone- big and small, made a habit of making jokes and pulling pranks. In earlier times, at this time of year in early April, in France, fishing was banned because it is the spawning of fish (the breeding season), had some idea as to jokes of fishermen throwing into the river herring. By doing so, they should perhaps exclaim: "Poisson d'Avril!" and the custom of the "fish of April" has remained.
Today, though we no longer have herring in freshwater, this "fish" still hangs, more discreetly as a small fish in paper on the backs of people who sometimes walk around all day (unknowingly) with this "fish of April" taped to their backs, making others laugh.
So, don't forget to check your back today... and Poisson d'Avril to you if you have succeeded in your joke for today!