Yep, it's officially October. That means a lot of things. Firstly, #SweaterWeather (get your chunky knits ready, because they're about to be your second skin). Secondly, boot season (from combat to Chelsea) is underway. And thirdly, it's time to start thinking about your Halloween costume. Natch, this is always struggle city – are you going to do a group 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!