There are several speculations as to how April Fool's Day came around, but here are the most likely theses:
One theory is that April Fools’ Day originates from Renaissance Europe. Some historians have linked the beginning of April Fools’ Day with the ancient Roman festival Hilaria, which means “joyful” and would have been celebrated at the end of March.
Other people think that April Fools’ originates from the 16th century when France switched from the Julian to the Gregorian calendar. The Julian calendar would celebrate the beginning of the new year on the 1st of April, while the Gregorian calendar celebrated its beginning on the 1st of January. Therefore, people who were slow at picking this up and those who celebrated the New Year later, were laughed at. People also stuck paper fishes on those people’s backs. In France that tradition has survived till the present day.
April Fools’ Day has come to represent joy and comedy, as well as the beginning of spring.