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!
Caroline Manzo’s Mashed Potatoes
Emeril Lagasse’s Garlic Mashed Potatoes
Mashed Potatoes with Ricotta