I, personally, like and use this crock pot/slow cooker version, as it frees up my oven for other dishes when making my holiday meals. Heating the prepared mashed potatoes in the crock-pot is a great way to keep the oven freed up for the other dishes.
Take the prepared mashed potatoes out of refrigerator about 3-1/2 hours before serving time. See Preparing Make-Ahead Mashed Potatoes above. Place the mashed potatoes in the crock-pot/slow cooker. Cover and use low heat for approximately 3 hours, stirring once or twice.
When you are ready to serve the mashed potatoes, stir, and then add some additional dollops of butter.