Opposite Day is an unofficial holiday when every action is modified so that meaning is negated. It is usually observed among children, and rarely lasts an entire day. Once Opposite Day is declared, statements mean the opposite of what they usually mean. Opposite Day can also be declared retroactively to indicate that what was just asserted had the opposite meaning of what was originally intended (similar to the practice of crossed fingers to automatically nullify promises). Paradoxically, the declaration that it is opposite day is negated as well, implying that it is not opposite day. Play has been compared to a children's "philosophy course".
In the first season of the cartoon SpongeBob SquarePants, an episode was titled "Opposite Day". In the episode the character Squidward attempts to use the holiday as a method of preventing his neighbors from intruding on the sale of his house.
In the "Full Court Dress" episode of Happy Endings, Max complains to Penny about having to babysit his niece and nephew, about the kids' "garbage hipster names", and about their parents' marriage being "doomed" anyway, which he is then revealed to be doing in front of his niece and nephew. In an attempt to cover, Penny tries to explain that Max didn't mean any of it, because it's "Opposite Day". Max then elaborates that their "dad doesn't have a secret Facebook profile under the name Jeffrey Scrotum."
In the Season 18 episode of The Simpsons, The Wife Aquatic, Homer is forced to work on a fishing boat to pay off damage he caused; when he accidentally beer-batters the bait and serves the bait to the crew, he says he did the opposite of what the captain ordered because it was Opposite Day, adding coquettishly "By the way, you're VERY ugly". When another crew member says they should NOT check the lines, the Captain angrily says "It's not Opposite Day!" and when Homer thinks that means that it IS Opposite Day, the Captain screams "IT'S NOT OPPOSITE DAY!"
BA Baracus uses this line in A-Team (2010) when Hannibal carjacks him in the Mexican desert.