You should never blindly believe something you read online, but on Saturday, you need to keep an extra eye out for fake news. It's April Fools' Day, a historic celebration where people around the world get pranked.

April Fools' Day may have begun as long ago as the 1500s, at a time when the French and other Europeans were changing to the Gregorian calendar, according the History Channel. It moved the start of each new year to Jan. 1 instead of in spring, but some people were left out of the loop, confused or frustrated by the transition. People still observing the new year in March and April were mocked, with pranksters sticking paper fish — in French, "poisson d'Avril" — on their backs as a joke.

Read: 5 Ways To Trick Someone With A Text Message

Fast forward 500 years, and the lighthearted holiday is now a seriously big deal.

Traditions vary depending on where you are. In Denmark, for example, you're likely to be pranked on May 1, or Maj-kat, according to Mental Floss. In Scotland, April Fools' is a two-day affair where people "hunt the gowk," meaning they send people on fruitless adventures.

The United States has gotten particularly into the April Fools' Day tradition, with hundreds of companies putting on elaborate pranks each year. In 1986, for example, New York City advertised an April Fools' Day Parade that never happened, CNN reported. In 1996, Taco Bell said it bought the Liberty Bell landmark. In 2008, BBC News revealed flying penguins left Antarctica for South America.

Read: 8 Prank Films To Watch April 1

"Companies feel enormous pressure to create these humorous videos, fearing that if they don't come out with one, they'll be considered unfunny or irrelevant," Alex Boese, of the Museum of Hoaxes, told National Geographic a few years ago. "I don't get sick of it at all. People love consuming humorous content, and April Fools' Day is a holiday specifically about this."