Your host put a lot of thought and work into organizing a formal dinner party. Show respect by dressing appropriately for the event. If you're unsure what your host means by "formal," ask her for guidance. Formal can encompass white and black tie, creative black tie and black-tie optional attire.
Most Formal Attire
White tie is the most formal dress of all and may only be expected at state dinners or, if specified, at evening galas and weddings. Men should be in tuxedo pants with a double braid, a dress coat, a stiff white pique wing-collared shirt, braces, cuffs and studs, a white vest and tie, black patent shoes and dress socks and gloves. Women should wear long, floor-length gowns and -- optionally -- gloves, which are removed before dining.
Black Tie and Creative Black Tie
Black tie is a step down in formality from white tie, but is still dressy. Men wear a tuxedo jacket and a black tie, but braces and vests are optional. Women may still wear floor-length gowns but could also opt for dressy cocktail-length dresses. Even a fancy little black dress or dressy separates will do. If the host specifies "creative black tie," a man can break away from the classic white shirt and black accessories to wear colors and patterns. Women may accessorize with whimsical, colorful bags, shoes and jewelry. A short cocktail dress in a glittery gold or silver, a dressy sweater with a ball gown skirt or tuxedo pants and a silk blouse are also appropriate.
Just "Formal"
The more vague distinction "formal" means you need to consider the time of year, your hosts, the occasion and the venue when choosing your clothing. If black tie isn't indicated on the invitation, a man can wear a dark, solid suit with conservative accessories. A formal gown may still be acceptable attire for a woman, but you may be better off with a dressy cocktail-length skirt or a little-black dress that you can accessorize to look more formal, if needed. Heels, dangly earrings and a glitzy clutch can dress up almost any look.
Dressing Don'ts
Men should skip white jackets, unless they are attending a daytime summer event or the invitation specifies white. GQ advises men to stay classic and not try to emulate celebrities who wear a dark suit with a black tie to a formal dinner party. Khakis and polo shirts are out of the question for men when "formal" is on the invitation. For women, casual flats or informal sandals are a definite don't. Save pantsuits for work and maxi dresses for walks on the beach.