Opposite Day is a make believe game usually played by children, the game has also been compared to a children’s “philosophy course”.

Conceptually, Opposite Day is a holiday where things are said and done in an opposite manner. It is not a holiday on any calendar and therefore one can declare that any day of the year 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). Although not an “official” holiday, it has recently become commonplace to celebrate Opposite Day (or Opposites Day) on January 25th of each year. 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. However, this issue can be rectified by acknowledging that when the statement “it is Opposite Day” is made, it is exempt from the normal system of reversal as its purpose is to inform others of Opposite Day, and to be better at speaking.

Links:
When is Opposite Day and Opposite Day Ideas