May 4 is considered a holiday by Star Wars fans to celebrate Star Wars culture, books 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. (Wiki)
Images above: The base of the Alliance to Restore the Republic located on Yavin - 4, one of the moons of Yavin vs. Tikal National park in Guatemala.