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.
French children fool their friends by taping a paper fish to their friends' backs.
Today Americans play small tricks on friends and strangers alike on the first of April. School children might tell a classmate that school has been canceled. Putting salt in the sugar bowl for the next person is not a nice trick to play on a stranger. College students set their clocks an hour behind, so their roommates show up to the wrong class - or not at all. Some practical jokes are kept up the whole day before the victim realizes what day it is.