For a more formal wedding, it is typically expected that female guests wear a long dress. But instead of breaking out the gown you wore to prom, try wearing a more appropriate evening gown. If the wedding is during the day, a pretty maxi dress should do the trick.
Is it appropriate to wear a long dress to an afternoon wedding?
Wear a short cocktail dress or dressy suit for an informal or semi-formal late afternoon wedding. For a formal late afternoon wedding, wear a dressy cocktail dress or a less formal long dress.