Preheat oven to 350 degrees with rack in lowest position. Place turkey on roasting rack set in a large roasting pan. Season inside of turkey with salt and pepper. Loosely fill neck and large cavity with dressing; fold skin over, and secure with skewers or trussing needles, if necessary. Bend wing tips forward, and tuck under neck cavity. Using kitchen twine, tie legs together securely. Rub turkey all over with butter; season with salt and pepper.
Add neck, giblets, and 3 cups water to roasting pan. Cover turkey loosely with aluminum foil. Roast 1 hour, and then baste with pan juices every 30 minutes, until an instant-read thermometer inserted in thickest part of a thigh (avoiding bone) registers 125 degrees, 1 to 2 hours more.
Remove foil; increase heat to 400 degrees. In a small bowl, stir together sugar and mustard; brush turkey with glaze. Rotate pan, and continue to roast turkey, brushing with glaze 2 to 3 more times, until thermometer registers 165 degrees, 45 minutes to 1 hour more (tent with buttered foil if browning too quickly; add more water if pan becomes dry).
Transfer turkey to a platter; reserve pan with drippings for gravy (opposite). Cover turkey loosely with foil, and let rest at least 30 minutes (or up to 1 hour). Before serving, remove dressing, and carve.
To ensure a juicy Thanksgiving turkey, buy an inexpensive instant-read thermometer, and roast the bird to the temperature specified in the recipe.