Mix Club Day is an anime only event and takes place in episode 70 of the original Prince of Tennis anime series.
According to Nanjiro's response in the episode, Mix Club Day has been an event at Seishun Gakuen since his days at the school over 25 years ago.
Mix Club Day is a self-participation event for the students in which they do not have to worry about grades or class as it is an all-day event. According to the students, on Mix Club Day students who are a part of a club are not allowed to participate in the events that their club is incharge of. Known participating clubs are basketball, soccer, baseball, volleyball, badmiton, tennis, ping pong, and curling.