April Fools' Day falls on the first day of April and it is usual to play practical jokes during the day.
People playing April Fool jokes reveal their prank by shouting April Fool when someone falls for their trick.
In the UK and some other countries, people are traditionally only allowed to play jokes on others until midday.
A person playing a joke after midday is the "April Fool" themselves.
It's also known as All Fools' Day.
In some other places like America, the jokes can go on all day.
In France if you become fooled on the April 1 you get called a poisson d'avril - which means April fish!
Have you been fooled?
There have been lots of jokes played on TV, radio and in the newspapers for April Fools' Day - and lots of people have fallen for them.
One TV prank included a BBC news programme making viewers believe that spaghetti is grown on trees.
Incredibly loads of people thought it was true.