You've answered your own question here. If you mean what attire is expected on formal nights: Tuxedo or dark suit & tie for men. Cocktail dresses for women........Wilba.
As long as you look smart it doesn't really matter. A long summer dress will look fine or a smart knee length dress.
A ordinary suit for men is fine, in fact I noticed on my last cruise that a large number of men wore no jackets and some no ties.