Cube your bread and place it in a very large bowl. Sprinkle salt, pepper, and sage over the bread cubes. Make sure you stir as you add the seasonings so they will cover the bread evenly!
Melt butter in a skillet or large saucepan. Add onion and celery and cook over medium heat for 5-10 minutes or till veggies are barely tender.
Slowly drizzle the melted butter mixture over the seasoned bread, stirring often so the butter covers evenly. Make sure mixture is well combined.
Pour into one large or two smaller casserole dishes or pans. Cover with foil and bake at 350° for about 20 minutes, or till heated through.