In sixteenth-century France, the start of the new year was observed on April first. It was celebrated in much the same way as it is today with parties and dancing into the late hours of the night. Then in 1562, Pope Gregory introduced a new calendar for the Christian world, and the new year fell on January first. There were some people, however, who hadn't heard or didn't believe the change in the date, so they continued to celebrate New Year's Day on April first. Others played tricks on them and called them "April fools." They sent them on a "fool's errand" or tried to make them believe that something false was true. In France today, April first is called "Poisson d'Avril." French children fool their friends by taping a paper fish to their friends' backs. When the "young fool" discovers this trick, the prankster yells "Poisson d’Avril!" (April Fish!)
It looks as though the Vatican had to straigten out the world way back then, and the French still didn't get it. Fools.