Yep, it's officially October. That means a lot of things.One of which, it's time to start thinking about your Halloween costume. Natch, this is always struggle city – are you going to do a ladies' costume with your baes (tip: Kendall, Gigi and Cara will win you major points) or something with the BF? Either way, work out why we celebrate Halloween with help from seven megababes in über-cool black gowns. Trick or treat, b*tches!