Worst Wedding Theme A cocktail dress is a shorter-length dress that ranges from just below the knee to 2 inches above the ankle (tea length) or touching the ankle (ballerina length). A dress can be worn to a semi-formal event. It is an evening dress that is identified by its length. It can be anywhere from above the knee to a few inches above the ankle. Cocktail dresses are worn for receptions, meetings and events that demand you to present in an outfit for the occasion. dresses must be without collars and sleeves. They are less elegant and less formal than an evening dress, but more elaborate and sophisticated than an ordinary one.