We are still uncertain about the origins of celebrating April 1st as April Fools Day or All Fools Day although it is now generally accepted that it originated in France in the 16th century.
The most widespread theory about the origin of April Fool's Day involves the Gregorian calendar reform of the late sixteenth century. Although popular, this theory has a number of problems with it. The theory goes like this: In 1582 France became the first country to switch from the Julian to the Gregorian calendar established by the Council of Trent (1563). This switch meant, among other things, that the beginning of the year was moved from the end of March to January 1. Those who failed to keep up with the change, who stubbornly clung to the old calendar system and continued to celebrate the New Year during the week that fell between March 25th (known in England as Lady Day) and April 1st, had various jokes played on them. For instance, pranksters would surreptitiously stick paper fish to their backs. The victims of this prank were given the epithet Poisson d'Avril, or April Fish. Thus, April Fool's Day was born.