Sautee the mushrooms, onion, celery, and salt in some butter or margarine. Cool and add the mayonnaise.
Mix the eggs and the milk in a separate bowl.Set aside,
Layer part of the bread cubes in a greased casserole dish.Top with half of the mushroom/mayo combination.Repeat this step, saving some bread cubes for the top of the casserole.
Pour the egg and milk mixture over the bread cubes and mushrooms.Spread the cream of mushroom soup on top of the casserole and finish off with the last of the bread cubes.Adjust the amount of bread cubes used throughout the casserole as you wish. Cover and refrigerate overnight.
Bake - covered - at 325 degrees for 60-70 minutes or until bubbly.Remove cover during the last 15-20 minutes.