Start by cutting the bread into 1-inch pieces. You can keep the crusts on.
Either a chef's knife or a serrated knife is good for this.
Crumble the sausage into a large skillet, and cook on medium until cooked through and browned in spots.
In a large bowl, whisk together the eggs, milk, salt, and dry mustard.
Place the bread cubes and sausage in a greased 13x9 inch dish.
Pour the egg mixture over the top (the cheddar will rise to the top). Refrigerate overnight (or at least 2 hours).
The next morning, preheat the oven to 350°F.
Bake for 45 minutes, or until the casserole is cooked through.