An American in Provence

April 1st, or le premiere d'avril is also celebrated in France.

From Wikipedia (french version)

"An April fish joke is done on the 1st of April of each year to friends and acquaintances. It is also custom to do jokes in the press, such as newspapers, radio, television, and on the internet.
For children, it consists of attaching a paper fish on the back of people one wants to mock. "April fish!" is also the exclamation as one is discovered, or to warn the victim of a joke, or that the story that has just been told is false.

Legend has it that this tradition originated in France, in 1564. Then, the year would have begun on the 1st of April, but the France king Charles IX decreed, by the edict of Roussillon, that the year would begin henceforth the 1st January following the Gregorian Calendar, marking the lengthening of the days, instead of at the end of March, (Solstice). Those who didn't follow the new calendar were called "fools" and sent invitations to fake parties and the like.

But indeed, the civil year never began on the 1st of April. If the exact origin of the usage of the fish remains obscure (perhaps because of the Christian symbol), the legend says that several of the subjects continued to celebrate around the 1st of April. Thus was born the fish, the April fish, the day of fools, the day of those that do not accept reality or see it differently.

It is worth noting that many different cultures have had days of foolishness around the start of April, give or take a couple of weeks. The Romans had a festival named Hilaria on March 25, rejoicing in the resurrection of Attis. The Hindu calendar has Holi, and the Jewish calendar has Purim. Perhaps there's something about the time of year, with its turn from winter to spring, that lends itself to lighthearted celebrations

This custom to do jokes is found in many countries, although not always with fish:
The British preserved their April Fool's Day (certain Scottish speak also Gowk or of Cuckoo), the Germans have their Aprilscherz, and this custom exists also in Belgium, Canada, Italy, the United States, Switzerland, and even Japan.

Nevertheless, the conjectures remain: was it to mark the end of zodiac period of Pisces, or to extend the period of lent, where it is forbidden to eat fish, or that one wanted to confuse the simple-minded with the offering of a fish? Or that fish are dumb and easy to catch?
In France, at the beginning of the 20th century, one sent oneself pretty post cards all decorated with fish. One wrote oneself, for this occasion, warm messages and wishes."