I think the answer is in the get ready to cruise section after you book the cruise. But from my Princess Patter last year it says
"During the day, sportswear and casual clothes are suggested.
FORMAL: A dinner jacket or dark business suit for gentlemen and a full length evening gown, cocktail dress or trouser suit for ladies.
SMART CASUAL: An open-neck shirt and slacks for gentlemen and a dress, skirt and blouse or trouser suit for the ladies.
Shorts and T-shirts are not permitted in the dining rooms."
Basically, wear the clothes you might wear to a wedding for formal evening, you don't have to wear tux and gown unless you want to.
A 7 day cruise should have two formal nights, your first day at sea will likely be the first one. If you really don't want to dress up, go to the buffet for dinner, but I think it is part of the overall cruising experience.