What is the best way to prepare mashed potatoes for 200 people for a sit down dinner? The day before the dinner I would peel the potatoes, cut into large chunks, boil until tender, and take off the stove.  I usually add some concentrated lemon juice or white wine vinegar to the water after the potatoes are cooked, but before draining for a few minutes just to help keep the potatoes from oxidizing or turning gray.  Drain the water off and place in a plastic container.  Let sit covered in a refrigerator or walk in. (A moist towel works well or once cooled, use plastic wrap.) The day of the event re-heat the potatoes in the oven at a low temp (300-325⁰F) then remove, rice or mash, add in the hot milk and butter, don't over mix.  Serve.