Mix together oysters, crumbs, milk, salt, pepper, butter, and optional celery. Butter a large baking dish and lay one haddock fillet in it. Brush with egg. Cover with a generous amount of dressing, and top with second fillet. Brush top fillet with egg and cover with remaining dressing. Bake at 375°F for 35-40 minutes, or until fish flakes easily with a fork.