Four days before service, place the salt, sage, thyme, black peppercorns, and allspice into a spice grinder and pulse until the peppercorns and allspice are coarsely ground, 5 to 6 pulses. Set aside.
Set the turkey, breast side down, on a large cutting board with the tail closest to you. Use an electric knife or heavy-duty kitchen shears to cut up one side of the backbone.
Turn the bird around and cut back down the other side of the spine. Reserve the backbone for stock. Discard any fat pockets or excess skin found inside the turkey.
Turn the turkey breast side up and use the heel of your hands to press down on both breasts, until you hear a cracking sound and the bird has flattened slightly.
Rub the seasoned salt on both sides of the turkey. Place the turkey on a parchment paper-lined half sheet pan, breast side up with legs running with the long side of the pan. Store, uncovered, in the refrigerator for 4 days.
Remove the turkey from the refrigerator and leave at room temperature for 1 hour.