Lobster Night is the second formal evening. On Zuiderdam, that is Thursday.
Dutch Night is one of our favorites. We really enjoy the Dutch Pea Soup; bami goreng; Dover Sole; and the "Biefstuk Met Champignons. Dutch Night is often when they have the deck barbeque.
I dont think there is a "best" night to go to Odyssey. We have dined there a number of times throughout four Zuiderdeam cruises and have gone on different nights each cruise.