Typically you will receive a formal invitation for these events that will state that black tie apparel is required. A black tuxedo coat, black trousers, white tuxedo shirt, black bow tie and matching cummerbund are expected. Women should wear a formal gown, preferably long. A cocktail dress should be reserved for a cocktail style function. This style of dress is usually reserved for very formal, typically official or government occasions.
Attire consists of a long tailcoat, usually white, but black is acceptable, with matching trousers.
Women are expected to wear a very formal, definitely long, gown. Always remove your hat when indoors. Even business casual means that your baseball cap stays at home. Hats should not be worn in restaurants, meetings, church, receptions or places of work. It is a sign of disrespect.