We flew home from Barbados on or around 20th December but as you're going to be on the ship so near to Christmas there should be a better atmosphere and some celebrations. Surely they wouldn't save everything for the passengers embarking on Christmas Eve (which I assume is the case).
I haven't looked in this year's brochure but if the ship is docked in Barbados for the whole day before you leave (which it was in our case) then an alternative to spending the night on the Marie Celeste would be to go into town on the last night. It's not far by taxi.
By the way, regarding your other post, I don't think this particular category gets many readers. The P&O website also has a very good message board and people are very helpful so you could find out more on that site. You can also arrange to meet people on board if you want!