For Christmas, ship had expected decorations. Lots of garland, lots of trees.
For New Years, first year we did it in atrium of Mariner. Normal New Years Eve decorations. It was a fantastic experience with all the free flowing chocolate and caviar. On Voyager, we were going through Canal on New Years Eve. Therefore, they had the party up on the pool deck. Only thing I remembered is that they had tons of streamers along with the required hats and noise makers. Although most found it a better experience, I still preferred the previous year on Mariner.
PARTY will be similar to our Voyager cruise as that also departed on 27 Dec. Mariner departed on 19 December so pax and crew knew each other much better by time New Years rolled around.
You will certainly have a wonderful time. We wish we could sail Christmas/New Years every year.