Yesterday was Halloween, and Hollywood took advantage of the holiday with parties last night and all weekend long! There were plenty of elaborate costumes at bashes from NYC to Las Vegas, with Britney Spears and Jason Trawick posing with her dressed up sons in London. Jennifer Garner got witchy in LA, where pregnant Hilary Duff and her husband Mike Comrie donned their robber finest. Jessica Simpson may have had the best ensemble of all, wearing a mummy suit to announce that she'll be a mommy and posing with her Larry Byrd fiancé Eric Johnson. Click through to see all the stars in costume this year so far!