There will be two formal nights. The first one is usually the first day after the sail date. The second one is on the next to last night of the cruise.
I have always brought my own tux, so I cannot help you there.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08