Butter a 9-x-13-inch baking dish or casserole dish. Arrange the bread in the dish, making two rows and overlapping the slices as needed to make them fit. Drizzle eggnog over the bread. Use the back of a spatula to press gently on the bread to even out the slices and help them absorb the eggnog. Cover and chill for at least an hour or overnight.
Preheat oven to 375°F and uncover the baking dish. Sprinkle with walnuts and sugar, and let the dish come to room temperature. Bake until heated through and the top is browned and crisped, about 35 minutes.