Stir Up Sunday doesn't really have anything to do with Christmas Pudding. In fact the name comes from the Anglican Church and is the last Sunday before Advent. The word stir up comes from the 1549 Book Of Common Prayer - 'Stir up, we beseech thee o Lord the wills of thy faithful people'
It has now become associated with stirring up the mixture for your Christmas pudding. Traditionally this was a big family event and jolly hard work mixing in all that dried fruit and other ingredients.
Each member of the family would take a turn at stirring (traditionally moving the spoon from East to West as that was the direction taken by the Three Wise Men on visiting the baby Jesus) and would make a wish at the same time.
Nowadays most people buy their Christmas puddings so this ancient family tradition is unfortunately fading away.