Star Wars Day may refer to one of two dates or events honoring or involving the Star Wars saga.1

May 4 is considered a holiday by Star Wars fans to celebrate Star Wars culture and honor the films.  It is called Star Wars Day because of the popularity of a common pun spoken on this day.  Since the phrase "May the Force be with you" is a famous quote often spoken in the Star Wars films, fans commonly say "May the fourth be with you" on this day.  

• 1. http://www.nbclosangeles.com/the-scene/archive/Forced-or-Not-Theres-a-Star-Wars-Day.html