Loads of girls in maxi dresses , sandals , denim jackets when i went last week but was very warm. I would go for jeans , t shirt and 3/4 sleeve jacket with flat shoes a must. (prob a waterproof rolled away in my bag just in case.
Are you sitting/standing? Comfortable shoes,layers, some kind of water resistant item if on the pitch area (a bin-bag really works). If you are being picked up, have them bring water,wipes and a fresh t-shirt