On the 1st of April in France they celebrate Poisson d’Avril. The idea of the day is to pin a paper fish on your friend’s back without them noticing.
Nobody knows for certain where this tradition has come from but it’s believed to date back to 1564 when Charles IX decided to move the beginning of the year from 1st of April to the 1st of January. In the days before phones and instant messaging, it took a long time for communications to filter through to everybody, and so the people in the countryside, that is mostly the uneducated folk, were the last to receive the news and so they carried on celebrating new year on the 1st of April. The upper classes mocked them for this, and started giving gifts on the 1st of April as a joke.
Because April 1st was often still during the period of Lent, meat was not allowed and so often fish would have been the main part of meal. For this reason many of the gifts were of fish, and over time the presents became more jokey and some people began to give fake fish on this day.
One of the best things about this time of year in France now, is that you can buy chocolate fish in the shops!
Related post: April Fools’ Day