Heat oven to 350°F. Dry bread cubes on large sheet 10 minutes, stirring once; transfer to large bowl.
Grease 9-by-13-inch dish with 1 tablespoon butter. Melt other 5 tablespoons in skillet; soften onion and celery 8 to 10 minutes. Add sage, thyme, salt, and pepper.
Toss vegetables with bread. Whisk broth and eggs; pour over and gently fold to moisten evenly. Stand 5 minutes.
Spoon loosely into dish without packing. Foil-cover; bake 25 minutes.
Uncover; bake 15 to 20 minutes until top golden and center 165°F. Stand 10 minutes.