"Opposite Day" is not an officially recognized holiday. There are some references to "Opposite Day" being on January 25. Generally, "Opposite Day" is simply declared on any day, and it begins whenever the starter says so.
"Opposite Day" has its roots in playground play, where a stubborn kid tries to work around being told no. On "Opposite Day" the meaning of words is the opposite; so, for example, no means yes on this day. It serves as philosophical development, as well as practice for standardized tests.
"Opposite Day" has appeared in a "Calvin and Hobbes" comic strip. It has also been mentioned on several television shows, including "Spongebob Squarepants," "American Dad," "The Simpsons" and "Robot Chicken."