Cut challah into 1-inch cubes and let cubes sit out in a bowl or on a baking tray, uncovered, for at least six hours to make them stale. (Alternatively, you can dry bread in a 250°F oven by laying cubes in a single layer on baking sheets and baking for about 30 minutes, or until bread is dry but not toasted (it should not start to brown at all).).
Preheat oven to 350°F.
In a large saute pan, melt butter over medium-low heat, then add onions and celery.
Add three teaspoons of salt and ground pepper to taste, and cook until onions are soft, about 8 minutes.
Add apples and herbs and cook until apples are soft, about 5 minutes more.
In a large bowl, combine challah cubes, cooked vegetables, and chicken broth.
Mix until the bread is saturated with liquid, and everything is evenly mixed.
Press stuffing into a 9x13-inch baking dish and cover with foil.
Bake at 350°F for 40 minutes; remove the foil, increase temperature to 450°F, and stuffing for another 10-15 minutes, or until the top starts to brown slightly.