Poisson d'avril should be translated as April Fish Day **. Generally the term is used to describe the French festival, aimed at having fun like the more common April Fool's Day. In the 600's the calendar that people used began on April 1, so people celebrated the new year on that day. There weren't enough days on the calendar to match the solar year, however, so the dates were all mixed up. When the calendar was changed over by Pope Gregory, the first day of the new year was January 1. Some people couldn't seem to keep it straight, so they would--like fools--celebrate the new year on the wrong day. Thus, April Fool's Day.
One very popular prank on Poisson d'avril is to make a paper fish and tape it to someone's back without
their knowledge; sort of like a "kick me" sign. (I won't even bother drawing a reference to being slapped with a mIRC trout). If you're caught with a fish on your back on April 1st, you become the source of much laughter and amusement. But exactly where the fish comes from is the cause of much speculation and various fish tales (pardon the pun). I've seen people explain that the fools were so out of place when they celebrated--doing it on the wrong day and all--that they were considered to be like
"fish out of water." Unfortunately, this is a common English expression and not very likely to have been the
origin of the reference.
Another explanation cites the French custom of exchanging gifts on New Year's Day. Since these fools
were celebrating on the wrong day, it seemed reasonable to give them gag gifts rather than real ones, so
people took to giving them silly objects, like fish, instead.
More likely is the idea that, since fish reproduce in early spring, an April fish is one that is fledgling and easy to catch.
**One can also be called a "poisson d'avril, in which case "Fish of April" is a more accurate translation.