It is called Star Wars Day because of the popularity of a common pun spoken on this day. Because 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.
-- per Wikipedia
Check out this newly released Star Wars themed book...
Star Wars: Vader's Little Princess
by Jeffrey Brown
In this irresistibly funny follow-up to the breakout bestseller Darth Vader and Son, Vader—Sith Lord and leader of the Galactic Empire—now faces the trials, joys, and mood swings of raising his daughter Leia as she grows from a sweet little girl into a rebellious teenager.
Smart and funny illustrations by artist Jeffrey Brown give classic Star Wars moments a twist by bringing these iconic family relations together under one roof.
From tea parties to teaching Leia how to fly a TIE fighter, regulating the time she spends talking with friends via R2-D2's hologram, and making sure Leia doesn't leave the house wearing only the a skirted metal bikini, Vader's parenting skills are put hilariously to the test.