There are many of us who want to do all we can on the day before the big cooking.
When I do mashed potatoes in advance, I peel, cut, boil, and drain them as usual.
Also as usual I add milk and melted butter
(you can add chicken broth instead, if you keep kosher), and whip them with my hand mixer. The experts say not to do that with a mixer, but I'm an expert and I always do them with the mixer. Anyway, I then put them into a buttered (or sprayed) baking dish or pan, put some more butter (or margarine - for kosher) on top, cover with plastic wrap and stick them in the fridge. Next day, I bake them for about 30 - 45 minutes just before serving, and stir in the butter or margarine I've put on top. I would not suggest using the microwave, as it will spoil the consistency. In the alternative, you could use those potato buds or flakes or whatever they are, and
do them just before the rest of your meal is ready. I don't care for their flavor, but there are many people who like them.
Good luck! Blessings to you - from Spike