Although the origins of April Fool’s Day are obscure, they most certainly have some connection to ancient Roman festivals which celebrated the first day of spring and the advent of planting season.
As Christianity spread through Europe following the collapse of the Roman Empire, some countries decided to make the first day of the year more significant by placing it on the Easter Holiday. As late as 1400, France still had Easter as the first day of the new year.
The British used March 25th (Feast of the Assumption) as New Year’s Day for centuries, typically following it with a week of festivals ending on April 1st. There is a 1766 reference to “people making fools of each other” at the end of the festival. Perhaps these festivals and an evolved “foolish tradition” is the source of April Fools Day.
The other popular theory relates to the Julian calendar, which was implemented by Caesar in 46 B.C. One of the changes he made was to move the first day of the year from March 21st to January 1st. Eventually all of the European countries adopted this change, but those folks who continued to celebrate the new year on April 1st would be labeled fools for continuing the old practice.