On this day in 1700, English pranksters begin popularizing the annual tradition of April Fools’ Day by playing practical jokes on each other. Although the day, also called All Fools’ Day, has been celebrated for several centuries by different cultures, its exact origins remain a mystery. Some historians speculate that April Fools’ Day dates back to 1582, when France switched from the Julian calendar to the Gregorian calendar, as called for by the Council of Trent in 1563. People who were slow to get the news or failed to recognize that the start of the new year had moved to January 1 and continued to celebrate it during the last week of March through April 1 became the butt of jokes and hoaxes. These included having paper fish placed on their backs and being referred to as “poisson d’avril” (April fish), said to symbolize a young, easily caught fish and a gullible person. Historians have also linked April Fools’ Day to ancient festivals such as Hilaria, which was celebrated in Rome at the end of March and involved people dressing up in disguises. There’s also speculation that April Fools’ Day was tied to the vernal equinox, or first day of spring in the Northern Hemisphere, when Mother Nature fooled people with changing, unpredictable weather.