Dress: You’ll want to wear a dress that is made of a refined fabric (like silk, lace or taffeta) and cuts around the knee. It should be much more formal than a sundress, but less formal than a ballgown. Most department stores and online retailers have sections devoted to party dresses, and any of those should do.
Suit: Most people wear dark suits to occasions with this dress code, but if the event is a daytime party, a light suit (beige, gray) is totally fine.
Dressy separates: If you wear a very snazzy top (think sequins or silk), you can probably get away with dress pants (say, velvet) or a skirt on the bottom.
Dress shoes, pumps
When in doubt, wear black.