Opposite Day (also known as Backwards Day) is a holiday where everything is the opposite of what it's supposed to be.
Opposite Day is sometimes declared retroactively to indicate that something which has just been said now has the opposite of its original meaning (similar to the practice of crossed fingers to automatically nullify promises).
Opposite day causes a self referential paradox. On opposite day, it could be said that "Today is opposite day." Therefore, it is not opposite day, but if you say it is a normal day it would be considered a normal day, which contradicts the fact that it has previously been stated that it is an opposite day.