Luther's plan wouldn't have worked anyway!
The Kranks' plan of going to the Carribean and skipping Christmas wouldn't have worked away, because you expect me to believe that a cruise ship travelling at Christmas wouldn't acknowledge it at all.
The ship would probably have decorations and a tree on board, and the captain would announce that it is Christmas when it happens. They can't escape it.
Besides, it wasn't like they were going to some Muslim or Jewish country. It is the Carribbean. I know many of the West Indian islands are Christian, so would celebrate Christmas.
Christmas is a worldwide event. Did the Kranks really think that they could run away from it completely?