"Formal night" usually means a tux or dark suit for the man and an evening gown or cocktail dress for the women. As for casual night, you can get by with a polo shirt and casual pants for the men and a sundress or light blouse and slacks for the women. The buffet restaurant will serve full dinners during the evening if you want to be casual.
By the way, I will be taking this cruise as my honeymoon. See you on board!