Why are they called plum puddings if there are no plums in them? Because they are wrapped in cloth, cooked for hours and then they sit 'plum' in their cloths!
Another bit of trivia: Why is it traditional to have at least 13 ingredients in Christmas Pudding? One ingredient for each apostle and one for Christ.
A Christmas joke comes to mind as I write this: What happens to you if you eat Christmas Decorations? You get tinsilitis!