Preheat the oven to 325 degrees F (165 degrees C). Grease a 9x13 inch baking dish.
In a large bowl, stir together the seasoning packet from the stuffing, hot water, and 1/4 cup butter until butter is melted. Stir in stuffing just until moistened. Let stand for 5 minutes.
Melt 3 tablespoons butter in a large skillet over medium heat. Add the celery and onion; cook and stir until tender. Mix in the cooked turkey, and season with celery seed, garlic powder, salt, and pepper.
Spread a layer of stuffing over the bottom of the prepared baking dish. Top with the turkey mixture, then spread a layer of mashed potatoes over everything. Cover with aluminum foil.
Bake for 15 to 20 minutes in the preheated oven, until heated through.