With nothing to do with horses and stirrups!
In Great Britain, the final Sunday of the Liturgical year, the Sunday before Advent begins, is "Stir-Up Sunday," taken from the Collect for the day from the Book of Common Prayer:
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 rewarded; through Jesus Christ our Lord. Amen.
Since a good Christmas "pud" needs to rest for some weeks before Christmas, it was traditional to bake the pudding on Stir-Up Sunday and then put it in a cool dry place until the holiday.
Stir Up Sunday and Christmas Pudding Traditions
Stir-Up Sunday and Christmas Pudding
John Lewis: Stir-Up Sunday