There's some uncertainty about when and where this bizarre tradition began, but the most accepted explanation traces April Fools' Day back to 16th century France. Up until 1564, the accepted calendar was the Julian calendar, which observed the beginning of the New Year around April. According to "The Oxford Companion to the Year," King Charles IX then declared that France would begin using the Gregorian calendar, which shifted New Year's Day to January 1.
Not everyone accepted this shifting of dates at the same time. Some believed that the dates should not be shifted, and it was these people who became the butt of some April jokes and were mocked as fools. People sent gifts and invited them to bogus parties.
Today in France, people who are fooled on April 1 are called Poisson d'Avril, which literally means the "April Fish." One common joke is to hook a cardboard fish to the back of a person. What a fish has to do with April Fools' Day is not clear.
Have fun and don't pull too many foolish tricks on your friends or families! I'm a little prankster, so I have to think of something good!