Mashed potatoes ARE one of those things you can make ahead for your holiday dinner. Prepare them as you always do and refrigerate, preferably in a glass microwaveable dish.
I've tried many ways of doing this. First, on the stove top. It is difficult to warm potatoes in a pot over a flame without scorching the bottom. Oven warming makes the potatoes a very strange consistency, not creamy like mashed potatoes should be. While microwaving is not always the best warming option for many foods, I found it to be the best for mashed potatoes.
Twenty minutes before you want to serve, begin microwaving (on high) your potatoes, covered with a paper plate or paper towel for 2 minute increments. Stir between. You may need to add a little milk if they're too thick. Continue microwaving until they're pipping hot. If you have a huge amount of potatoes, divide them among smaller bowls.
I found this to be the best way to warm cold mashed potatoes. Anything you can make ahead of time makes for a much less stressful holiday dinner.
Santa will soon be on his way!