Butter a 9″-x-13″ baking dish and arrange bread slices in an overlapping layer.
In a large bowl, whisk eggs, eggnog, vanilla, cinnamon, and nutmeg until combined. Pour mixture over bread and press bread down, making sure all pieces are submerged in eggnog-egg mixture.
Cover baking dish with aluminum foil and refrigerate, 3 hours, or up to overnight.
When ready to bake, preheat oven to 350°. Transfer dish to oven and bake, covered, until cooked through, 25 minutes.