Poisson d'avril is the equivalent of April Fool's Day but not quite. It is "April's Fish Day"... Today, people will be playing tricks ("jouer des tours à quelqu'un") on others, the most popular one being discreetly sticking a paper fish on their victims' back! But all other kinds of false stories and traditional tricks are allowed too.When the fish or trick are found out, they shout out "Poisson d'avril!" at their victim. If you managed to trick somebody, you would say "Je t'ai attrapé(e)" ("I caught you!").That could be a possible connection with "le poisson".
Some books offer obscure reasons why the day is assocciated with fish; one suggests that François, duc of Lorraine, held prisoner in Nancy by Louis XIII, escaped "le 1er avril " by jumping in a river and swimming to freedom!
But there are more theories, such as the Reformation of the French calendar by Charles XIV. You'll find more explanations in French on l'Internaute.
So as well as being busy cutting out paper fish, the French will be busy "faire des blagues" , "faire des calunars","faire des farces" (playing tricks) hoping that their victims will have "le sens de l'humour"!
