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Opposite Day is a game usually played by children. One can declare that today is Opposite Day (sometimes retroactively) to indicate something which will be said, or has just been said should be understood opposite to its original meaning (similar to the practice of crossed fingers to automatically nullify promises).
An analysis of this concept would conclude that Opposite day causes a self-referential paradox. In theory, the statement "it is opposite day", if uttered on opposite day, should mean "it is not opposite day". However, the statement "it is not opposite day" also does not clearly communicate the meaning of "it is opposite day", since it must first be communicated that it is opposite day before the statement can be interpreted this way. Therefore, there is no unambiguous way to communicate that the current day is opposite day.