Hermione had kept her own outfit a secret, even from Ginny.  Of course, Ginny had pouted.  Harry and Ron were upset she was "ditching" them.  She reminded them that hardly ever again would she get a chance to be this anonymous in any part of the world.  Sure, she could go out disguised in public, but for one night it was a perfectly acceptable and encouraged choice.  She was going to take advantage of it.  