How the dates are worked out
We start from dinner, take away 20 minutes for the turkey to stand and the longest roasting time in USDA's 325 °F chart for that weight, and round down to 5 minutes. That's when the turkey goes in. Then we go back 4 lb per day in the fridge (USDA: about 24 hours for every 4 to 5 lb) to get the latest day to start.
The “USDA fridge days” column is what FSIS's own thawing calculator prints for each weight. Our planner reproduces it for every weight from 6 to 36 lb.
Starting earlier is fine up to a point: USDA says a thawed turkey can stay in the refrigerator for 1 or 2 days before cooking. If you brine, add the brine time. The planner checks that wait for you. Sources: Turkey Basics: Safe Thawing (opens in a new tab) and Your Safe Thanksgiving Guide (opens in a new tab).