If you're in orchestra level, it's probably not going to relax. If you're up with the peons and tourists, worry less. I wouldn't think the season makes a big difference for the dress code I was there in October, so it was a little cool, and wore a long shirt (long johns under), plain knit black top, and black wrap, and mary janes. Not dressy by any means, but not touristy. If I'd had nicer pants that looks reasonable and weren't denim or the REI hiking style, I would have worn them. Men could probably get away with similarly comfortable but not denim pants. No shorts, obv. Probably no leggings on women unless the tunic you have over them is really nice. The Italian couple in the same box (for reference) had on a suit and nice dress but not ball gown-ish. There are probably enough out-of-towners there that they don't judge openly. What they say behind our backs is another matter, but that probably happens anyway. :)