Time Out says
This is the largest Halloween street party in the world, and there's really no better place to be on October 31st. Sure, the crowd is huge (like, a half million people huge) and a bit belligerent, but the amazing display of costumes and general merry-making spirit deem it at least a worthy stop, if not your main destination for the evening. There will be dancing, drinking, and many mid-parade costume contests. Even if you don't plan on entering one, it's best to still come dressed to the nines—no one likes a party pooper in jeans and a t-shirt.
See photos from the 2015 West Hollywood Costume Carnaval.