This is easy for guys - wear a Tuxedo! But for women, it's a bit more complicated. You can wear anything from a long elegant gown to a dressy mix and match outfit to a formal cocktail dress. Just imagine you were attending the Oscars; no big deal!
An easy tip to figure out what to wear - dress according to what you expect the host to wear. Attending a black tie holiday party for New Years? Then wear a long gown. But if it's a work function, a girl can't go wrong with a classic LBD accompanied with a statement necklace and makeup.