If you received a formal invitation, and especially if the event benefits a charity or association, consider it a dressier affair. Choose dress and skirt lengths from mini to just above the ankles. If you're unsure how formal the event is you can always ask a friend who is also going and see what he plans to wear and use that as a starting point.
Women:
Choose fabric according to the season. Wool and wool blends are perfect for fall and winter; satin, silk, rayon and fine-gauge knits are great for the spring and summer months. A black wool dress with stockings and pumps is a simple solution for fall or winter. Pair a strapless or spaghetti-strap dress with an embroidered cardigan and slingback pumps or strappy sandals for a spring or summer event. If the invitation came by phone or e-mail, chances are it's a more casual affair. Match hair adornments and a handbag to the motif, material or colors in your outfit. Show off your gold charm bracelet or favorite pair of dangling earrings.
Men:
Dress up casual business attire for a more formal after-work cocktail party. Stick to dark, muted tones in heavier fabrics for fall and winter gatherings. Wear lighter-weight fabrics and brighter colors for spring and summer get-togethers. Go dressy business casual for a more formal late-afternoon gathering; opt for a suit in the evening. For a less formal event, wear a nice casual outfit in the afternoon and dressier business casual clothes in the evening. Treat cocktail parties on weekend afternoons less formally than those scheduled in the evening.