I need to rewatch, but it crosses over to midnight on Christmas Day at some point, doesn't it? I think one of the people on the Titanic looks at a watch. Certainly if the Doctor expects people to be Christmas shopping, then it (not the whole episode, but the scene when the Titanic passsengers teleport down and the Doctor speaks to Wilf) not ON Christmas Day, because the presents would have been bought and people off at home.