Set a rack inside a large heavy roasting pan. Season turkey lightly inside and out with salt and pepper, then with spice mix, massaging it into the skin. Transfer turkey, breast side down, to prepared pan and refrigerate, uncovered, overnight. Remove turkey from refrigerator; let stand at room temperature for 1 hour. Preheat oven to 375°F. Mix celery, pepper, and onion in a medium bowl. Fill turkey cavities with vegetable mixture, scattering any remaining vegetables over bottom of roasting pan. Brush turkey with oil. Roast turkey, basting occasionally, for 1 hour. Using paper towels, flip turkey. Roast, basting occasionally, until an instant-read thermometer inserted into thickest part of thigh registers 165°F, 1-1 1/2 hours longer. Transfer to a platter. Let rest for at least 20 minutes before carving.