How long should I cook a Christmas ham?
Hi -- what size is the ham?
And how are you preparing it?
And what type of ham?
If it is the basic hams sold today by most companies it has been pumped with water, salt, sugar, and probably sodium nitrite, sodium phosphate and sodium erythorbate. These hams have reached a temperature during processing that kill trichinia and other pathogens, at least 154 degrees F by USDA regulations. The hams are cooked. This goes with boneless or bone-in. The standard reheating procedure is to put about 1/2" water in a pan and heat the ham, covered, in a 325 oven for 20 minutes per lb. The last 1/2 hour, uncover the ham and glaze to ones preference at a 400 oven. My family use to produce York style hams (somewhat similar to today's style ham, except better) and this was the standard instructions we gave all customers for over 50 years. No one ever complained.
Cooks Illustrated has a method for a supermarket ham that is already fully cooked, to wit using one of those oven roasting bags, which really does keep the ham from drying out. You can remove it at the end if you want to glaze the ham.