Their are dozens of alleged origins floating around explaining April Fools day, but very few of them have any solid historical evidence. Of the many proposed origin stories, however, the one linking of modern April Fools day celebrations with the ancient Roman tradition of Hilaria is one of the stronger.
In Roman tradition Hilaria festivals were celebrated on the vernal equinox to honor the goddess Cybele–“The Great Mother”, worshiped in Rome as a source of Roman power and military success. Romans rejoiced that the days were, after the gloom of winter, again longer than the nights and held eight days of celebrations.
The final day of the celebration was the Hilaria–traditionally on March 25. All throughout the day all sorts of amusing games and pranks as well as other forms of entertainment were undertaken. The celebration culminated in an elaborate masquerade where Roman citizens went disguised as other Roman citizens–it was perfectly acceptable to imitate people of power and prestige, engaging in light hearted pranks and mocking along the way.
What’s interesting to note is that the whole concept of foolishness and pranking around the advent of spring is certainly not a Roman invention. World wide there are examples of similar behavior around the vernal equinox. In Iran, for example, Iranians play pranks on each other on the 13th day of the Persian New Year which routinely aligns with April 1st. It would seem that a good prank or two is a perfect way to burn off restless energy stored up over the winter months.