if I am making mashed potatoes the night before- what is te best way to prepare/reheat on christmas day- so they taste like I just made them?
I use a double boiler to bring them back to serving temperature. You might want to whip them quickly w/ a whisk to get the consistency correct.
If you try to use direct heat in a sauce pan the potatoes will scorch.
I can't vouch for it, but a lot of Chowhounds have reported that keeping them warm in a slow cooker on low works.
"what is te best way to prepare/reheat on christmas day- so they taste like I just made them?"
They can be held or reheated in a double boiler or a crockpot, but the only way they are going to taste/have the texture of being just made is for you to make them when they are about to be served.
If I HAD to make potatoes the day before (Id find a work around honestly) I would leave the cream and butter out the night before and simply gently mash the potatoes, then heat the cream and butter and stir them into the potatoes as you are warming them over low heat right before service.