Hi there cruiser, Princess does have two dressy nights on all 7 night or more sailings.My husband and I do not pack formal clothes or his suit jacket any more. I wear dress slacks and a sparkly top and he wears slacks and a long sleeve collared shirt for all our nights on board. We were on Sky Princess for Xmas and outfits ran the gamut. Silly Santa suits, elf costumes, a few tuxedos and mother of the bride type dresses.
I would dress the way you are most happy on a night out to a restaurant and you can’t go wrong.