In the Star Wars universe, BBY stands for "Before the Battle of Yavin". The Battle of Yavin is the climactic battle that occurs at the end of "A New Hope," where the Rebels launch a successful attack on the Death Star over Yavin IV and destroy the Empires superweapon, kicking the Galactic Civil War into high gear. The BBY system is used to measure time in the Star Wars universe, and it counts down, with 0 BBY representing the year in which the Battle of Yavin occurred. Therefore, when a date is given as BBY X, it means that the event occurred X years before the Battle of Yavin.