April 1 in the U.S. is celebrated as "April Fools' Day."  It's a day on which people traditionally play funny tricks and pranks on each other.  The pranks can range from the simple to the incredibly complex and well-planned, but they're almost always ended with the prankster yelling, "April Fools!"

Some newspapers and companies make an art of the April Fools' Day prank, leaking false information or writing fake articles as a joke. The Museum of Hoaxes has a list of the top 100 pranks of all time, including my personal favorite:
The British astronomer Patrick Moore announced on BBC Radio 2 that at 9:47 AM a once-in-a-lifetime astronomical event was going to occur that listeners could experience in their very own homes. The planet Pluto would pass behind Jupiter, temporarily causing a gravitational alignment that would counteract and lessen the Earth's own gravity. Moore told his listeners that if they jumped in the air at the exact moment that this planetary alignment occurred, they would experience a strange floating sensation. When 9:47 AM arrived, BBC2 began to receive hundreds of phone calls from listeners claiming to have felt the sensation. One woman even reported that she and her eleven friends had risen from their chairs and floated around the room.