BBY/ABY is used to position items along the Star Wars Timeline. BBY refers to Before the Battle of Yavin. ABY refers to After the Battle of Yavin. The Battle of Yavin in Episode IV: A New Hope occured in 0 BBY/ABY.
Although the Rebel Alliance has won a few battles against the Empire, hope is fading. The Empire is about to finish building the greatest weapon the galaxy has ever seen—the Death Star. The rebels' only chance to defeat it now lies in the unlikely hands of a princess, a scoundrel, and a farm boy. . . .