Green flag --- That's right, it's April Fools Day. In France, people celebrate this event by making paper fishes and sticking it to peoples back. This tradition dates back to 1564. In 1564, King Charles IX of France set the start of the year from April 1st to January 1. There were no technology for communication which made lower class people living in rural areas the last to hear the news. Some people didn't hear the news for a few years, and some continued to still celebrate the new year in April instead of January. Those people are called "Fools". The "fools" were the recipients of pranks and had jokes played on them. People who are fooled on April 1st are called "Poisson d'Avril" (April fish)
In French, it's called, Le poisson d'Avril (The fish of April/April fish).