Using a mixer, mix all ingredients together until well-combined and fluffy. Place in a 9 x 13 baking dish and cover with foil. Freeze or refrigerate (for up to one day) until the day you want to serve them.
Warm to room temperature before baking (or increase baking time by 15 minutes if they are a bit cold.) If you have frozen them, place in the refrigerator the day before to thaw.
Bake at 350 degrees for 30 minutes.
Divide recipe by 2 or 4 for smaller amount.
Notes
My family of 7 eats about ¼ of this recipe, which is the amount shown in the picture. I love to buy potatoes when they are on sale, make the entire recipe, and then freeze it in four containers.
Recipe by The Gingham Apron at https://www.theginghamapron.com/grandma-bettys-make-ahead-mashed-potatoes/