You can peel the potatoes the day before, then cover with cold water and refrigerate until ready to cook. Alternatively, you can cook and mash the potatoes up to 2 days in advance. Let them cool to room temperature, then transfer to an airtight container and refrigerate. Just before serving, pour a thin layer of milk onto the bottom of a wide, heavy pot. Add the mashed potatoes, set the pot over medium heat and stir occasionally until the potatoes are hot.