Sunday best, for example, sundresses, blouses and nice jeans or a blouse and a skirt for women. And a button up or pearl snap and nice jeans for men. I would recommend not wearing stilettos, or any heels that may sink in grass, as the ceremony is taking place in a field :)
I would recommend bringing both sunscreen and mosquito spray. Both will already be there, however, it’s always better to be safe!