Answer:
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.)