Two days ahead, discard lumps of fat and the metal or plastic leg truss, if any, from turkey. Rinse turkey and pat dry. Reserve neck and giblets (but not liver), chilled, for Roasted Turkey Gravy. Set turkey on a rimmed baking sheet. Slide hands under breast skin as far as you can go to loosen it. Turn turkey breast down and slide hands under thigh skin to loosen. Rub 1/4 cup salt under breast and thigh skin and inside cavities. Turn turkey breast up. Cover loosely with plastic wrap and chill 1 day.
The next day, turn turkey over and drain any liquid from pan. Cover again, this time with 2 clean kitchen towels, and chill 1 day.
Two hours before roasting, remove turkey from refrigerator and pat dry.
Preheat oven to 400° with a rack set in lower third. Combine softened butter with chopped thyme and sage; season with 1/2 tsp. each salt and pepper.
Patting turkey dry as needed, rub two-thirds of herb butter under skin of breast and thighs (access thighs from back), under neck flap, and all over outside of bird. Dollop remaining butter in cavity and add thyme and sage sprigs. Sprinkle outside of turkey with remaining 1 tsp. pepper. Tuck wings under turkey and tie legs together with kitchen twine. Fold neck skin under body and secure with a skewer.
Set a V-shaped rack in a large roasting pan and set turkey breast up on rack. Add carrot, celery, and onion to pan. Roast 30 minutes. Reduce oven to 325°, baste turkey with melted butter, and put reserved turkey neck and giblets in pan. Roast, basting every 30 minutes (use pan drippings once butter is gone), until an instant-read thermometer inserted into thickest part of breast to the bone reaches 165°, 2 to 3 1/4 hours more.
Lift rack with turkey and tip juices from cavity into pan. Transfer turkey to a carving board, tent loosely with foil, and let rest in a warm place about 30 minutes. Reserve vegetables, turkey neck and giblets, and pan drippings for gravy.
Set turkey on a platter and tuck in sprays of herbs and crabapples.