We sailed on December 26 - January 2 on the Victory this past Christmas. There was a big New Year's Eve party on the Lido deck. Champagne, music, dancing, midnight toast, etc. They even set up a big screen and were showing Dick Clark's New Year's Eve show (though it wasn't hosted by Dick Clark!) I think just about everyone on ths ship was on the Lido deck that night. At dinner that night, they had party favors and hats and everything at all of the tables. It was really fun.
The temperature was cooler, (70s-80s as the other poster said). Definitely warm enough to wear shorts and go swimming. It was nice not having stifling heat. It did get a little cold at night up on the Lido deck, with the wind blowing.
I think this cruise was about the most expensive cruise of the year when we were looking at prices. Flights really weren't too bad (price-wise), but expect to pay more than just about any other time for the cruise.