April Fool's Day is a holiday on April 1. The day involves pulling pranks on other people, followed by a "April Fool's!" after the victim gets pranked. The episode "Fools in April" is based on this holiday.
Role in episode
Squidward, fed up and annoyed by SpongeBob's excessive laughter on this holiday, pulls an April Fool's prank on him by squirting mustard on the floor for him to clean up. Once SpongeBob goes to clean it up, his feet are caught in a lasso which Squidward cuts the rope of, sending SpongeBob flying across the Krusty Krab and bumping into several obstacles, ending up in a trash can.
SpongeBob runs off crying and the Bikini Bottomites turn on against Squidward for pulling such a cruel prank.
Squidward pulls off many unsuccesful attempts to apologize to SpongeBob later on, and eventually does so as long as it be kept a secret between the two of them. However, SpongeBob then opens the door to reveal that everyone witnessed his apology and says along with them, "April Fool's!" Squidward then reveals that he was lying about feeling sorry for SpongeBob and says "April Fool's!" himself, running off hysterically.