We were on the Zuiderdam for Christmas last year and it was wonderful! Especially enjoyed the International Choir on Christmas Eve. Christmas morning there were announcements about Santa being seen in the vicinity of the ship and a build-up for the kids about Santa's anticipated arrival with gifts for them. It was a very festive week on board. Docs usually arrive 3-4 weeks in advance and included is lots of info about your cruise. Boarding usually starts around 11:30 a.m. (your docs will probably say 3 p.m., but HAL instituted early boarding a while back, so disregard the later time). If you do choose to board late morning, your cabin may not be ready and you'll be invited to visit the Lido for lunch and wait for an announcement that cabins are ready (normally around 1:15 - 1:30). The lunch buffet is excellent, so do try to make it.
We enjoyed last year's Christmas cruise so much we booked a 14-day sailing of the Rotterdam for the holidays this year. Will be our first visit to the Panama Canal and we're really looking forward to it.
Have a wonderful cruise!