All the cruises we have been on have a suggested/recommended dress code for each night and we have never seen it enforced .having said that most people follow it but for whatever reasons (lost luggage) you may not have all the clothing and as you have paid good money to eat in the main dining room so go there,sod the snobs .
Casual: sundresses or slacks and blouses
Smart Casual: Dresses or pantsuit
Formal: cocktail dresses
Casual: sports shirts and trousers
Smart Casual: slacks, shirt and jackets
Formal: suits and ties, or tuxedos
If you are dining in the dining room then you are expected to dress accordingly - you can always eat in the buffet area on this night or have room service.