"So I have tickets to the opera in three weeks and I'm quite excited!! But I need help with clothing selection..."
On any trip we need to know about conditions, timing, goals of the trip, etc. We need more information to help you.
Now, keep in mind that I think humanity is going through de-evolution; not to mention the accelerating decline of western civilization -- all of this demands that only formal wear be considered for the opera. But given the poor state of affairs, let's consider some items:
Where is the trip? New York, Boston, Chicago, San Francisco, LA?
Is it a "real" opera or something like a rock opera?
Is it Opening Night? Is it (god forbid) a matinee?
What is your goal? To be seen, to get laid, or to actually enjoy the opera?
Okay... lots of permutations here, so let's look at the "biggies."
Opening Night. Formal only. Preferably a designer gown by Dior or similar. Bob Mackie is acceptable, but Bob doesn't do a good job of sewing on buttons - so avoid any of his gowns with buttons. Your escort must wear a tuxedo.
To be seen. Formal still works best here, unless in LA or at a rock opera.
Getting laid. If that is NOT your goal, please get a custom gown by Jason Wu. .38 cuben might be your best bet to get laid -- unusual and transparent. Make it revealing.
Okay... let's say you actually just want to enjoy the opera and comfort is paramount. There are still some guidelines, and all operas require garments with an acceptable label. All designer-labeled casual wear will work with the following exceptions:
Boston: Nothing from Eddie Bauer or LL Bean. Nautica or Tommy Hilfiger are better. No denim.
New York: Designer labels only. No outdoor company labels. Denim okay if the garment retails for over $332.18.
Chicago: No outdoor company labels. Denim NOT okay.
San Francisco: Designer denim preferred. Outdoor labels are a go especially Patagonia and the Dead Bird Company. REI, LL Bean, and Eddie Bauer are out.
LA: Anything goes. Bizarre trumps fashion.