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
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.
4th Grader: IT'S OPPOSITE DAAAYZORZ!!
Smart Person: If it's opposite day, than that means it's NOT opposite day, dumbass.
4th Grader: OMG WTF YOU MEEN NERRRD! HAHAIDIOT.
by A guy. April 1, 2005
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
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
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
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
(march 27th! or so i've heard hehe.)

a day when "no" means "yes" and "shut up" means "keep talking" and etc..u do the opposite of what someone tells u to do.say the opposite things.blah blah blah.
a conversation between a girl & her boyfriend when theyre hugging on opposite day:

Girl: dont kiss me
Boy: *kisses her right on the lips*
by chass i licious July 7, 2005