Casual attire is the norm by day. At night, smart casual attire is enforced in all dining rooms, which includes at minimum, sun dresses for ladies, and long pants and collared shirts for gentlemen. Carnival permits dress shorts in the dining room.
All cruise lines (except NCL) have at least one formal night per cruise, where more dressy attire is required. Suggested wear for ladies is a cocktail or evening gown; for gentlemen, a dark suit. The choice of how dressed-up is entirely yours. If you prefer to dress casual, the buffet restaurant always allows regular shorts and t-shirt.