为什么圣诞节叫Mary Christmas?

Let's say that on December 20 you were to meet a friendly space alien.That is,let's say that his space ship discreetly drops him off in your back yard while you are looking out your window.You walk outside to meet the visitor,and you find out he's a pretty nice guy.His name is Gorg,he is wearing a costume that makes him look passably human,he speaks reasonable English,and he explains that his goal is to spend a week on the planet to learn about its people.He asks if you would consider being his guide for the week,and you decide to take on the job.