Today is "Stir up Sunday". That is the day when Xmas puddings were traditionally made. The name comes partly from that and partly because the Church of England prescribes prayers for each Sunday of the year and the one for the 5th Sunday before Xmas begins
Stir up, we beseech thee, O Lord, the wills of thy faithful people; that they, plenteously bringing forth the fruit of good works, may of thee be plenteously reward
It was the tradition that every member of the house took a turn at stirring the pudding so it had to be done on a Sunday when there was no work or school. Small coins or tokens (today it would be a 5pence coin) would be added to the pudding. It would then need cooking for 8 hours today and to be heated for a further 2 hours on Xmas day when it would be eaten with cream or custard or brandy sauce.
For any of you who want to try here is a link to a recipe
http://britishfood.about.com/od/christmas/r/xmaspud.htm
The brandy is optional and can be omitted.
The suet used in the pudding is nowadays usually vegetarian but originally was beef suet which is the fat which surrounds the kidneys.
Perfectly good puddings can now be bought in any supermarket and can be heated on Xmas Day by 5mins in a microwave. You may guess whether most people buy readymade or cook from scratch.