1. May the Fourth be with you. Celebrating the Star Wars film franchise, Star Wars Day is observed by fans on May the Fourth, an obvious pun on the popular Jedi phrase "May the Force be with you." Taking puns a step further, May 5 has come to be called "Revenge of the Fifth," a play on "Star Wars: Episode III - Revenge of the Sith," a day for fans to join the "dark side" and celebrate the Sith lords from the Star Wars movies.
2. Fully fan inspired. While Star Wars Day is a tribute to the movies made by George Lucas, Lucasfilm had nothing to do with the holiday’s creation or its ongoing observance.
3. London calling. The phrase tied to Star Wars Day was first used May 4, 1979, when Margaret Thatcher took office as Prime Minister of the U.K. Members of Thatcher’s Conservative party published full-page advertisement in the London Evening News congratulating the new Prime Minister, which read: "May the Fourth Be with You, Maggie. Congratulations."
4. Kicked off in Canada. The first organized Star Wars Day events were held at the Toronto Underground Cinema on May 4, 2011. Sean Ward and Alice Quinn organized the celebration, which included an Original Trilogy Trivia Game Show; a celebrity judged costume contest and Star Wars tribute films and parodies screened.
5. It’s a Small World After All: The Walt Disney Company, which purchased Lucasfilm in 2012 has officially observed the holiday since 2013 with special Star Wars events May 4 at both Walt Disney World and Disneyland.
Sources: https://en.wikipedia.org and http://www.starwars.com/