Opposite day was first introduced on an episode of "SpongeBob Squarepants" This concept has taken off since then. Every day there is always gonna be some smart ass kid who declares it opposite day.
It is opposite day, which means everything is the opposite of what it usually is today, which means today would actually be the opposite of opposite day which would mean that it is actually not opposite day at all. Opposite day has turned into a punchline for smart asses everywhere for example "your not ugly...on opposite day tehehe" it is set up much like the "not" jokes on Borat. Will the popularity of opposite day eventually fade? Well the awnser to that question depends if today is opposite day or not.