Gently loosen the turkey skin on the breast by sliding your hand between the skin and bird. Insert pieces of butter between the skin and breast.
In a medium bowl, toss together the celery, bread cubes, and onion. Season with garlic powder, salt and pepper. Stuff this mixture into the cavity of the turkey, and place the apple halves inside with it. Place the turkey into a roasting bag, and pour the apple juice over the inside and outside of the turkey. Close the bag, and place turkey into a large roasting pan breast side up.
Bake for 3 to 3 1/2 hours in the preheated oven, or until the internal temperature is 180 degrees F (85 degrees C) when measured in the meatiest part of the thigh. Open the bag, and remove turkey to a serving platter. Let stand for 20 minutes before carving.