If you make this gravy the day before Thanksgiving, you'll be much more relaxed on the holiday.
Make giblet broth: In a large saucepan, place neck, giblets, celery, carrots, onions, bay leaf, parsley, 10 cups water, and 1/4 teaspoon pepper; bring to a boil over high heat. Skim; reduce heat, and simmer 1 hour. Strain broth (if less than 6 cups liquid, add enough water to make 6 cups); discard solids. Set aside. (To store: Cool to room temperature, transfer to a container, cover, and refrigerate up to 1 day.)
Transfer turkey from pan to a serving platter. Place pan on stove across two burners. (If using a disposable pan, pour drippings and browned bits from pan into a medium saucepan and proceed). Over medium-high heat, bring drippings to a boil; cook, scraping up browned bits, until syrupy. Add cider; cook, stirring constantly, until syrupy, 6 to 10 minutes. Add flour and cook, stirring until incorporated, about 2 minutes.
Stir in giblet broth, a little at a time. Bring to a simmer, stirring constantly, until gravy reaches desired thickness, 15 to 20 minutes. For a smoother consistency, strain gravy through a fine-mesh sieve, if desired; discard solids. Season generously with salt and pepper. Keep warm; whisk well before serving.
Instead of homemade giblet broth, you can use 2 cans (each 14.5 ounces) chicken broth plus 2 1/2 cups water.
To make Cider Gravy as prepared on "The Martha Stewart Show," use 1/3 cup flour instead of 1/2.