Universal strictly bans guest costumes and costume masks from the event. With costumed actors in the streets, the last thing the park wants are guests getting confused. Universal will still allow T-shirts and other themed items.
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And, yes you can bring your own Harry Potter robe from home to the theme parks. You will of course still have to follow the safety guidelines when it comes to wearing your Harry Potter robe on some of the rides at Universal!
For safety, masks and clothing that drags on the ground are prohibited, but additionally, Guests aged 14 or older can't come in costume. That doesn't mean you can't have fun with how you come dressed to the parks.