You probably already knew this. I thought this explanation was well done.
First of all, you have to understand that it is not the letter X that is put into Christmas. We see the English letter X there, but actually what it involves is the first letter of the Greek name for Christ.
See the full blog post here:
Why is X Used when it Replaces Christ in Christmas? by R.C. Sproul | Ligonier Ministries Blog.
