You'll probably end up seeing some sun when you first show up, especially if you arrive early. Do your skin a favor and throw on a hat and a little sunscreen to avoid any accidental damage or burning. Most concert goers prefer caps, but a cute fedora or floppy summer straw hat will do.
Cobalt blue jeans and a vintage T-shirt may look cute, but they're going to be very comfortable if your concert goes on in warm weather. Go for something flowing that fits the evening breeze. Throw on a strapless mini dress with a loose-ish tiered bottom for form-fitting appeal, or go all festival-chic with a black spaghetti-strap maxi dress.
There's nothing more frustrating than thinking your hobo bag is the perfect accessory until you're jumping up and down to music at a concert while trying to clutch the bag close to your body with your elbow. Even worn cross-body, large bags are sort of a stumbling block to a good time. Go for a mini-bag instead, like a travel-style purse that has just enough room for the essentials. If you're feeling brave, go hands-and-shoulder-free with a modern fanny pack.
If it's crazy hot or if a sudden downpour takes over, the last thing you want is to have to carry your shoes in hand. Flat, strappy sandals will keep your feet from getting too stuffy and they'll dry easily.