A make-ahead recipe for mashed potatoes is Do-Ahead Mashed Potatoes from Betty Crocker. You need potatoes, milk, heavy whipping cream, salt and pepper, and butter or margarine.
Start by scrubbing 9 medium potatoes and removing the eyes, although peeling them is optional. Chunk the potatoes into pieces, and place them in large pot with 1 inch of boiling water. Add enough water to just cover the potatoes, and bring the water back up to a boil. Reduce the heat, and continue cooking them covered for another 20 to 25 minutes. Drain the water, and keep the pan over low heat, shaking the potatoes regularly to help dry but not cook them.
Remove the potatoes, and add 3/4 cup of milk, 1/2 cup of heavy whipping cream, 1/2 cup of butter, 1 teaspoon of salt and 18 teaspoon of butter to the saucepan. Turn the heat to medium-low, and stir the mixture. Once the butter melts, reserve and chill 1/4 cup of the mixture, and pour the rest into a bowl with the potatoes. Use an electric mixer to mash the potatoes into a fluffy texture, and then transfer them to a baking dish. Either bake the potatoes at 350 degrees Fahrenheit for 40 to 45 minutes with the reserved milk mixture, or refrigerate the potatoes for later.