I went to my cousins wedding last weekend. It was outside and super casual. This dress was so comfortable and though you can't it, there's a small, gold exposed zipper in the back. It also comes in about a dozen different colors.
Dress: H&M; Belt: H&M; Sandals: Charlotte Russe; Jacket: H&M
(Didn't realize til just now as I was typing this, that basically everything I wore was from H&M)