The success of the Christmas roast, or any roast, for that matter, depends largely on cooking it to just the right internal temperature. Your most accurate guide to doneness, points out the National Live Stock and Meat Board, is a roast meat thermometer. Place the thermometer in the center of the largest muscle so that the bulb does not rest in fat or on bone.
For beef or lamb roasts, cook to 140 degrees F. for rare, 160 degrees F. for medium and 170 degrees F. for well done.
All fresh pork roasts should be cooked to the well done stage of 170 degrees F.
Fully-cooked hams need only be heated through to an internal temperature of 130 degrees to 140 degrees F. while cook-before-eating hams should be cooked to 160 degrees F.
To allow for a rise in internal temperature during standing time, remove the roast from the oven when the thermometer registers about 5 degrees F. below the doneness desired.