I would wear something comfortable and simple since you'll probably be doing a lot of walking--whether its to concessions or the bathroom. I'd also bring along a cardigan/sweater/jacket because 53 feels chilly to me.
What about your favorite jeans or flowy skirt and a rock T-shirt with a cardigan over it? I also like to bring a cross-body messenger bag to these sorts of events for carrying essential items or storing extra layers. Are you in to hats? I think a lightweight fedora would look cute and help keep you from getting sun-fatigued if its going to start in the afternoon.
Sounds like it will be a great concert!