I did a QM2 10-night Caribbean cruise, where we had the Black & White, a costume ball and one other I can't recall. They are formal nights. The B&W was a formal evening where the "theme" was for everyone to dress in black and White. Most men wore tuxedos and women were in cocktail dresses, both long and knee length. In the evening, after dinner, was the Ball, where there was dancing in the Queens Room Ballroom. It was all very elegant, with a wonderful live band playing a lot of 1940s era music, but other kinds of music as well.
The other formal nights were pretty much the same formula, dressing for dinner and an evening of entertainment and dancing.