Everyone should have their staple go- to suit, tuxedo, gown and/or little black dress.
Men, it is preferred that you wear a tuxedo. If you choose not to wear a tuxedo, the only other acception is to wear a dark suit (acceptable colors, black, grey or navy) with a tie and your best dress shoes. Tuxedos no longer consist strictly of bow ties and a cummberband. In fact, JUST SAY NO!
Woman should wear a formal gown or a formal cocktail dress that falls no shorter than above their knees.