On one of our back to back cruises, our second sailing was the first with Christmas decorations. (This was a long time ago -- when you only had to go to Guest Services to check back in, which is why we were hanging out in the atrium.) As the last guest left, the atrium becomes quite a workplace. It was fascinating to watch how quickly they put everything up. They didn't miss a beat between when one group of guests left and being ready for the next group. As others have said, the large gingerbread house is the last thing constructed -- I believe not only because it takes a long time and special skills but also because they want to let the guests take part in watching that come together.