Although it's not a national holiday, April Fools' Day is widely recognized as a day to play practical jokes and invent elaborate hoaxes. In other words, it celebrates foolishness.
This day of pranks and trickery has been observed for centuries, but its origins are unclear.
The most popular theory is that France altered its calendar in the 1500s so the year would begin in January to coincide with the Roman calendar. Previously, the start of the year had been recognized at the beginning of spring, around April 1.
Word of the calendar change traveled slowly though, so many people who lived in rural areas continued to celebrate the beginning of the year in spring.
As the story goes, these people were often referred to as "April fools." They were mocked by having paper fish stuck to their backs and being called "poisson d'avril," or April fish.
The fish is said to symbolize an easily caught fish, or a gullible person, and the tradition continues today.
Staff at French restaurants in Canada sneakily attach 'poisson d'avril' on April Fools' Day. (Photo: N Wong/flickr)
Although this is the most widely accepted explanation, Alex Boese, curator of the Museum of Hoaxes, who's studied April Fools' Day's origins, disagrees with it.
"[The French] theory is completely wrong because the day that the French celebrated the beginning of the year legally was Easter day, so it never really was associated with April 1," he told National Geographic.
Boese believes the holiday simply grew out of ancient European spring festivals of renewal, in which pranks and disguising one's identity are common.
Historians have linked April 1 to ancient festivals such as Hilaria, which Romans celebrated at the end of March by donning costumes.
Foolishness around the world
Nonwestern cultures also have traditions that resemble modern-day April Fools' Day.
India celebrates Holi, a spring festival in which people play jokes and throw colorful dyes at each other. And in Iran, the holiday of Sizdahbedar, which typically coincides with April 1, is observed by playing pranks.
It's also been proposed that April Fools' Day is tied to the vernal equinox, when Mother Nature fools people with unpredictable weather.
Today, people, companies and media outlets often stage elaborate pranks to amuse and trick the "poisson d'avril."
One of the most famous pranks occurred in 1957 when the BBC broadcast a story about the Swiss Spaghetti Harvest.
The fake video showed Swiss farmers picking fresh spaghetti, and the network was flooded with calls from people interested in purchasing spaghetti plants.
Pasta harvest photo: Getty Images