I bought a spiral cut ham after Thanksgiving. It has a sell by date of Dec. 16. Is it still safe to cook for christmas?

You can reach out to the company on the label and they will be able to give you more info.

Sell by <> expiration HOWEVER it also is certainly an indicator. Is the ham wrapped and has been refrigerated? Is it cured? That will extend its life as well. You should follow your nose when you open it but honestly if in doubt throw it out. What you DON’T want it to give everyone food poisoning.