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1.
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
 
2.
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. March 31, 2005
 
3.
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 07, 2005
 
4.
(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 07, 2005
 
5.
Opposite day isnt a real day, where you dont do the opposite. People will say nothing opposite. Every other day of the year IS an opposite day.

Oh, and you musnt be reading this on opposite day, or youll understand it. :( (and yes, the smiley isnt opposite)
Guy 1: Hai its opposite day
Guy 2: U LIAR ITS NOT NOW YOU SAID... BUT THEN IT IS! BECAUSE I SAID ITS NOT! ZOMG!!!!!1!11!!!
by W (my name starts with it) June 25, 2011
 
6.
Celebrated on April 30th, the opposite of April 1st. The day when you are nice to your boss, teacher, enemy and can be a complete Ahole to your best friends. You can just say the opposite of what you want to say, and confuse/laugh at all the people who dont know why you are saying what you are saying.

Because April fools, has become a cliche.

Also spelled yad etisoppO, and occipatai
Beanerous Brokus I -Did I ever tell you that you look great.

Guy - You muckfluck its opposite Day.
by Beanerous Brokus I July 20, 2006