Evening dress code per HAL's web site:
Evening dress falls into three separate categories - casual, informal and formal. Each night, a daily program will be delivered to your stateroom announcing the suggested dress for the following evening.
Comfortable, relaxed clothing is fine for evenings designated as casual; however, T-shirts, jeans and shorts are not allowed in the dining room or public areas. During informal nights, dresses or pantsuits for women and jackets (tie optional) for men are standard.
On festive formal evenings, women usually wear cocktail dresses or gowns and men usually wear business suits or tuxedos. There are approximately two formal nights per week. (Gentlemen: Though business suits or tuxedos are suggested attire for formal evenings, they are certainly not required. You are welcome to wear a jacket and tie on formal nights.) Formalwear for ladies and gentlemen can be pre-ordered for your use for the duration of your cruise. Just call Cruise Line Formalwear at 800-551-5091 to reserve and it will be ready in your stateroom when you board.
In order to complement your fellow guests, Holland America Line asks that you observe the suggested dress code throughout the entire evening