Black Tie Optional
This is still super formal, so either wear the same attire you would to a black tie affair or wear the fanciest cocktail dress you can find. If you go the cocktail dress route, make sure there are rich details like beading on the frock and that you are strutting your stuff in your favorite stilettos.
A.B.S. by Allen Schwartz Strapless Cocktail Dress, $390; lordandtaylor.com