The New Year was celebrated between March 25 and April 1 by the French when the Julian calendar was still in use. However, in 1564, after the introduction of the new Gregorian calendar, the festivity was moved to January 1. While most people quickly adapted to the new calendar because it was proposed by the church, some people resisted the change, and they subsequently became victims of elaborate jokes, hoaxes, and pranks. These people were invited to New Year’s parties on the 1st of April only to show up and find that there was no party. 

They were derided and referred to as “the April fools.” That’s how we ended up celebrating April Fools’ Day on April 1st.