If you are on the Sun Princess, it is probably at ten day cruise. There are two formal nights on a ten day cruise. The information posted by other responders is all correct. The Sun Princess seems to encourage getting really dressed up. It's lots of fun! Dressy pants...........and a dressy top will suffice..........sparkles are even better. If you want to keep packing light, you could bring a long skirt or pants, and two different tops, or a scarf/jewelry, to change the look. I dont wear high heels, on a ship, but lots of women do.................. You may wish to bring a pashmina or shawl, as it is sometimes chilly from the a/c in the dining room and theaters. My husband wears a tux, and about 33-50 percent of the men do, but the others wear a suit.......... There are always a few that do not dress up. The Princess Patter (onboard daily newsletter) posts the dress code for each evening.........mostly "dressy casual" with the two "formal" nights.
Have a great time!