Hence April Fool's Day

"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. In the Julian Calendar, as in the Hindu calendar, the new year began with the spring equinox around April 1.

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 and were called 'April fools.'"