What do I wear to the NYC premiere of Harry Potter and the Deathly Hallows part 2?

I won tickets to go and watch the red carpet, and then the screening. I want to be comfortable...there will be a lot of walking/standing, humidity (not to mention sitting on the bus from Boston) ...but I don't want to look under-dressed. I'm thinking a sleeveless dress with medium height strappy black heels, and putting my hair up (that part's not a choice...if it's not up, it will get frizzy/bushy). Thanks