Pure, sincere, refined, true
The opposite (antonym) of opposite is same.
opposite of opposite to opposite from
An opposite of the word "to" is from.
If you have to be opposite on opposite day, that makes it a day, and then there was no opposite day. But that can't be true since you HAVE to be opposite, which means that you have to be normal even though there is an opposite day because you are supposed to be opposite on opposite day, which means that you act normal as if it was a normal day, because that is the opposite of opposite day. (This is a classic "liar's paradox". Since there is already an opposite day, you should have a normal day because that is the opposite of an opposite day. But then it is not an opposite day.)
You could say that you are being "opposite of how other people are" on Opposite Day. If you say the opposite of an opposite, it is a true expression.
The opposite of up is down. So, opposite of the opposite of up is up itself.
The opposite of the opposite is the original value, -9.
The opposite of 'hi' would be 'bye'.
The opposite of from would be to, or toward.