Opposite day is any given day where whatever
one says, in truth one means the opposite. Usually this is not planned
ahead of time, but declared loudly at a random
point, particularly when declaring so can be used to one's advantage
Billy: Can I see your Boba Fett? I will give it back.
Bobby: Okay *hands it to Bobby*
several minutes pass
Bobby: Give it back!
Bobby: You said you would!
Billy: *clutching Boba to his chest* Nu-uh! Today is opposite day!
There are rules for opposite day that can vary, such as declaring a "time out" when one gets too confused with using negatives, and if you are tricky, you can say that the declarer of opposite day is in error, for in truth they would have had to say that it is not opposite day, but this logical paradox is usually not discovered until at least 3rd grade, or sometimes by the more advanced 2nd grader.
A favorite among elementary schoolchildren, and the immature everywhere. Also useful if you wish to negate something embarassing just said.
Also see: Inside-Out Day