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See opposite day.

1. A term used to describe that it is, in fact, opposite day which can only be used on opposite day. The

"not" is usually over-emphasized, and the term usually follows a wink, nudge or other non-verbal que that the speaker means the opposite.
The rational behind this use of the term is that if it is Opposite Day, then the correct way of proclaiming that it's Opposite Day, is by actually proclaiming how it isn't Opposite Day.
This is the most commonly used form of the phrase.

2. A term that is used to genuinely describe how it is not opposite day. This use of the phrase is usually used by those that do not plan on partaking in Opposite Day, and therefor will mention that it's actually "not Opposite Day" when people say it is (or on Opposite Day, isn't). This usually does not work, as the pro-Opposite Day party acts as if the other party is in cahoots with them, not shutting them down.
1. "Guess what today ISN'T. Today is NOT Opposite Day!" *wink* *nudge*

2. "Stop saying that it's opposite day, because it's not Opposite Day." ... "That's not what I meant, I'm not agreeing with you in Opposite language, I'm trying to tell you that it isn't opposite day!" ... "UGH!"
by ItsGordie February 26, 2010
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Rooted in playground tradition. A fictional holiday, in which everything you do or say means exactly the opposite. Often used at the end of a sentence to negate what you just said.
"You look nice.... on Opposite Day!"
by Dr. Michael yates January 17, 2005
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A fake holiday where you do exactly the opposite of everything.

Obviously is wrong in itself, because if everything is opposite, than opposite day cannot exist. And no, it can't go the other way, obviously, since there is no opposite day.

Try this logic with 4th graders.
Smart Person: If it's opposite day, than that means it's NOT opposite day, dumbass.
by A guy. April 1, 2005
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Opposite Day is a fun "game" played on another. You never know when this day takes place, but when it does, everything you say has the opposite meaning of what it would on a normal day. The quest is on to figure out when this day actually happens, so you can safeguard yourself from its horrors.
H-bird: Imma snaxe you, you snaxe
*readies sticky hand*
Snaxe: Haha, It'sssss Opposite Day today, you can't snaxe me!
H-bird: Gosh Dangit.
by Hecretary Bird July 27, 2020
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A day where everything you say means the opposite of what it normally would. Kinda juvenile, but its fun too.
OMG It's opposite day, so I'm gay, Tony Blair is a fascinating and compelling leader, and my g/f is flat...
by theoneandonlyhazer July 8, 2005
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Opposite Day (n): Today is opposite day, so you must say no for yes and yes for no. It's also the only day you go back and stay back.

Opposite Day is the day where everything is reversed. But if opposite day is opposite day then shouldn't opposite day be opposite to opposite day? Meaning that everything that is opposite is opposite because opposite is opposite of opposite
2nd Grader: Hey student(teacher in this case), did you know today is opposite day.
Teacher: No, I didn't. But if today is opposite day little Timmy, shouldn't it be opposite?
2nd Grader: Wtf does that say?
by SenshiButaniku April 25, 2019
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When it’s like how it is, but opposite.
Aviel: *hitting Ethan with a belt*
Ethan: *blindfolded*
Matthew: It’s like Opposite Day!
by Sanjobanjo August 4, 2019
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