Semi-formal from Emily Post: "A semi-formal dress code is best described as being more formal than what you would wear to an office, but slightly less formal than what you would wear to a formal event. Do keep in mind that if the event is later than 6 in the evening, you’ll want to err on the side of formality. Women Short afternoon or cocktail dress A “little black dress” Long dressy skirt and top Dressy separates Men Dark, business suit Matching vest (optional) Dress shirt Tie Leather dress shoes and dark dress socks"