It's kind of weird because episode 13 is generally the Halloween episode but no one has mentioned Halloween. Instead, the Liars are crashing the Ravenswood-party-that-requires-a-costume-and-is-also-in-a-graveyard. The girls are all, "WTF. Who throws a party in a graveyard?" and the answer is apparently, "Ravenswood" and not, "well, who goes to an Adam Lambert murder Halloween train party, GIRLS?" or "who actually attends a party in a graveyard."
BUT ANY EXCUSE FOR GORGEOUS GIRLS IN COSTUME!
Sweeney: Gorgeous girls in gorgeous, elaborate period costumes. TV teenagers have the best closets.