Preheat the oven to 350 degrees F (175 degrees C). Prepare mashed potatoes according to package directions. Set aside.
Melt butter in a large skillet over medium heat. Add onion and celery, and cook, stirring until tender. Stir celery and onion into the mashed potatoes, then mix in the eggs. Break the toasted bread into small pieces, and stir into the potatoes. Season with salt and pepper, and transfer to an 11x7 inch baking dish.
Bake for 45 minutes in the preheated oven, until heated through, and the top is golden. More butter, salt and pepper may be sprinkled over the top if desired