Usually the formal nights are on evenings when you haven't spent the day in port. On your particular cruise, I don't know. However, I do know that Princess does not follow their standard guidelines on Alaskan cruises except for the number of formal nights. They show the other nights as either semi-formal or casual. On our Dawn Princess cruise last summer, there were no semi-formal nights. Fortunately I knew from previous cruisers and could pack lighter but for some reason, Princess would not change their information to reflect this. I would have been quite annoyed if I had packed some semi-formal clothes if it wasn't needed.