In a large mixing bowl, whisk together the eggnog, eggs, vanilla extract, ground cinnamon, and ground nutmeg until well combined. This will be the custard mixture for the French toast.

Place the slices of day-old bread in a shallow dish or baking pan. Pour the eggnog custard mixture over the bread, making sure each slice is fully coated. Allow the bread to soak in the custard for about 5 minutes, flipping the slices halfway through to ensure even absorption.

While the bread is soaking, preheat a large skillet or griddle over medium heat. Add a pat of butter to the skillet and let it melt, coating the surface evenly.

Carefully transfer the soaked bread slices to the preheated skillet, working in batches if necessary to avoid overcrowding. Cook the French toast for 2-3 minutes on each side, or until golden brown and crispy.

Once cooked, remove the French toast from the skillet and transfer to a serving plate. Repeat the cooking process with the remaining soaked bread slices, adding more butter to the skillet as needed.

Serve the eggnog French toast warm, garnished with a sprinkle of powdered sugar, a dollop of whipped cream, and a drizzle of maple syrup. Enjoy the festive flavors and the comforting texture of this delightful holiday breakfast!