Hi, I think it is ok. Xian is not as cold as before and some other northern Chinese cities. Wear warm clothes and that will be fine~

Hi, I think the most important thing is not about temperature but it will be very dry at that time. So please be prepared:)

I don't think Dec. is not a pleasant time. There will be fewer tourists in winter. Also, you don't need to worry rain during travel. And the whether won't impact any interest here.