Whether you rice 'em or smash 'em, the best way to keep mashed potatoes warm all day long on Thanksgiving is in a bain-marie (a heated water bath). Simmer water in a large pot on low, and place a smaller pot (covered) full of the mashed potatoes inside the larger one. They'll stay perfectly warm until it's time to serve!
Emeril Lagasse’s Garlic Mashed Potatoes
Mashed Potatoes with Ricotta