Unlike some holidays, the history of April Fool's Day (sometimes called All Fools' Day) is not totally clear (it resembles Valentine's Day in this development). There really wasn't a "First April Fool's Day" that can be pinpointed on a calendar. Some believe it evolved simultaneously in several cultures from celebrations involving the first day of spring.
The closest point in time that can be identified as the beginning of this tradition was in 1582, in France. Prior to that year, the new year was celebrated for eight days--beginning on March 25. This celebration culminated on April 1. With the reform of the calendar under Charles IX, the Gregorian Calendar was introduced and New Year's Day was moved to January 1. This paralleled the Roman Catholic Church's declaration of January First as important in the life of Christ.
However, communications traveled slowly (e.g. by troubadours or messengers on horseback), and many people did not receive this news for several years. Some who were more obstinate refused to accept the new calendar and continued to celebrate the new year on April 1. Such recalcitrant folk were labeled by authorities, or a more "enlightened"
populace, as "fools." They were ridiculed, often sent on "fools' errands," and subjected to other practical jokes.
This harassment evolved into a tradition of prank-playing on the first day of April. The tradition eventually spread to England and Scotland in the eighteenth century. It was later introduced to both English and French colonies in the Americas. April Fool's Day thus developed into an international joke extravaganza, with different nationalities' populaces practicing regional humor. Pranks performed on April Fool's Day range from the simple (e.g. Someone saying, "There's a bug on your shoulder!") to the elaborate--creating an entirely false scenario for a "fool" believe. Putting salt into the sugar bowl of a coffee drinker with a sweet tooth is a common practice. Whatever the prank, the trickster usually ends it by shouting with glee at his victim, "April Fool!"
April Fool's Day is a "non-serious" observance. One doesn't buy gifts or treat loved ones to scrumptious meals. There are no "cancellations" of school, work, or mail service. It's simply a holiday on which one must maintain "Constant Vigilance," for one may be the next April Fool!