There are two formal nights on a 7-day cruise. Usually on the evening of Day 2 and Day 5. If you do not wish to dress in formal attire, then you may choose to eat at the buffet where it is always casual. For the main dining room, dress is formal on these nights. Enjoy your cruise!
Just returned from the Island Princess. Formal is the dress code for both Traditional and Personal Choice seating on the second and fifth evening.
If you want casual you'll have to eat in the Horizon Court Buffet, which okay for a quick snack, but no where near the same food in the dinning rooms.