I’ve done ‘make ahead’ mashed potatoes many times. Just boil them, mash, add butter and milk (or cream!) and put them aside in the fridge until the next day.
Then, all you have to do is heat them as a casserole in the oven (or in a crockpot). You might want to stir in a little more butter and cream, if they’ve gotten dry.
That is basically the make ahead mashed potato recipe I found online. It’s piled high after mashing, with sour cream, butter, and cream cheese. Then fridged. My experiment will be in using non dairy ingredients. I actually went to Whole Foods and bought vegan cream cheese and vegan sour cream. Ugh. And for the butter I will use olive oil in smaller quantity. It might end up a disaster.
(In case anyone is wondering why the vegan, we keep kosher and can’t have dairy with meat, and you gots to have the homemade turkey gravy on the potatoes plus you want them next to the turkey, don’t you?)