Add butter, celery, and onion to a large skillet and saute over medium-high heat until butter is completely melted and vegetables are very tender.Stir in bread cubes and sprinkle with Italian seasoning, garlic powder, and salt and pepper to taste. Continue to stir over medium-low heat until bread is moistened and hot throughout (3-4 minutes).Taste, add salt and pepper to taste if needed.Stir in any optional add-ins if desired. Use to stuff a 10-12 pound turkey or cover and keep warm until ready to serve as-is.