Every year on April 1st people from around the world try to trick, fool and prank each other just for the fun of it. It's a day celebrated worldwide by pranking and/or humiliating another person.
Celebration differs from country to country. In France, Italy and Belgium people try to tape a picture of a fish on a persons' back without them noticing and yell "April fish" when it's discovered. In the UK, Australia, New Zealand and Zimbabwe people only pulled pranks until noon, but this tradition is slowly disappearing.. In Scotland the its called "Hunt the Gowk Day" and people send the victim around delivering an "important" message.
In my country people try to trick each other into believing or doing something ridiculous and/or untrue. When the person falls for it, the one playing the prank yells "Aprili li li". The phrase has no meaning as far as I know. Aside from that, people, usually children, dress up in costumes.
So how did this fun day come to be? When and why did people start playing pranks on each other and trying to trick each other? Well, it's origins are very uncertain and no one knows for sure.
One theory is that it began in Roman times. You see the ancient Romans, Hindus and other ancient cultures celebrated the New Year around April 1. In 1582 the Gregorian calendar was introduced and it was decided that New Year's Day will be celebrated on January 1. Some people refused to accept the new calendar and continued celebrating New Year on April 1, while others simple didn't learn about it. Those who knew and accepted the change started making fun of these people by tricking them into believing ridiculous things, sending them on "fool's errands" etc. Eventually it spread across Europe.
The problem with this explanation is that April Fool's Day was already well established in England long before they adopted the Gregorian calendar in 1752. There are also no direct historic evidence.
Nowadays even the media gets in on the fun. News papers and TV stations try to pass ridiculous information as legitimate. Some even work together to make it more believable. You can check out the Top 100 April Fool's Day hoaxes of all time. Also see Google's April Fool's jokes.