I am of the mindset that seeing a Broadway show, or any show of that caliber, is a special thing, regardless of how often you go. And so one should dress accordingly.
I usually wear jeans, (nice jeans, not ripped or dirty of course) either a tee shirt from, or relating to, the show or a nice top, and comfortable (not dirty) shoes. (It is New York after all.) I don't notice how people dress unless they're in sweats or a costume (neither of which is acceptable.)
KAD, I see where your coming from yet I disagree. I find that something would feel more like an event if it is something that you don't do very often. If, for example, seeing a Broadway show is a once a month activity for a family then I can see how that would be considered an event. But, the feeling of an event would get lost on me if it is something that goes from being something special to being a regular weekend activity for me.
I don't dress like I just rolled out of bed when I go to the theatre. But, on the other hand, I don't go out of my way to look nice either.
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