Grease a 9x13-inch baking dish generously with butter or cooking spray.
Arrange bread cubes evenly in the prepared baking dish.
In a large bowl, whisk together eggs, eggnog, milk, sugar, vanilla, cinnamon, nutmeg, and salt until well combined.
Pour the egg mixture evenly over the bread cubes, pressing down gently to ensure all pieces are soaked.
Drizzle melted butter over the top, cover tightly with plastic wrap, and refrigerate overnight (or at least 4 hours).
On Christmas morning, preheat oven to 350°F and let the casserole sit at room temperature while oven heats.
Remove plastic wrap and bake uncovered for 45-50 minutes until golden brown and puffed, and the center is set.
Let cool for 5 minutes, then dust with powdered sugar and serve warm with maple syrup.