April Fools' Day is celebrated on 1 April in many countries around the world, though often under different names (for example, in France they talk about the 'April fish'.) Te tradition is always the same though; you play tricks on people, usually by trying to get them to believe something that isn't true.
In fact the original day was on 25 March; the Roman festival of Hilaria, which means laughter. A typical example of this was an official-seeming announcement that tickets were on sale for citizens to go to the stadium and see the lions being washed.
Later in the Middle Ages, the date became 28 December, the day of the Feast of Fools (still practised in Spain) which was very similar to what we have today. References to 1 April start appearing from around the 15th or 16th century.
You can learn more about the history of April Fools' Day and examples of past tricks here.