An April fool in France is called an April fish - why? No one is really sure.
'April Fool's Day' supposedly started in France according to the French (this is so typically French you can't believe it) when one of the kings (Louis? Henri? I get them all mixed up) decided that the calender year would now start in January and that all over France it would be The Same for everyone. (Before, apparently, each region had a different New Year's Day, most being on the first of April. Well, everyone accepted January 1st, but there were (there are always) those who resisted change and hung on stubbornly to their day...The first of April. And those people were made fun of by others. And this developed into the April Fool's day.
"Hey you fool, stop celebrating today - New Year's day was three months ago!"
Now, a little harder to explain is the fish. In France, on April fool's day, children cut out a little fish in paper and tie or tape it to some unsuspecting soul's back. They then spend the rest of the day giggling about it. Why a fish? Some say it's a cross-over from Easter coming soon & the sign for Christians(Christ - Fish - April Fool's fish) But most people just think that of all the animals a kid could draw and cut out easily and quickly - a fish was the easiest. The person with the fish on his back was ridiculed all day long (The Fish fool!) and thus 'Poisson d'Avril'.
Now, in Germany, they also have April Fool's day, but there they call the day by the name of an animal - any animal at all you can imagine...But not a fish. It's never Fischtag in Germany. Hundtag, pfordtag, gantzertag...you name it, but not a fish. (and please pardon my german spelling)
At any rate, Happy April First!