Actually it would be recommended! I went on a cruise with Carnival not knowing and felt very foolish! If you can't find a suit a dress shirt and dress pants would be fine!
You will need at least a pair of slacks & a nice shirt. Formal night is always optional. You don't need a suit & tie as long as you are reasonably dressed. They know that not everybody can bring one. Keep in mind that its Christmas Week & many will dress with that idea. Many guys take the jacket off & put it over the chair. They only wear the jacket for the photos. Look nice & you will be fine.
If your are going with some people and have the dining plan cruises normally have 1-2 Formal Dining nights so yes bring a suit. Or ask whoever made the cruise reservation to find out for you. Have Fun!!!!!!!!!