Let us start off with the origin of the word Christmas:
"The word for Christmas in late Old English is
Cristes Maesse, the Mass of Christ, first found
in 1038, and Cristes-messe, in 1131"
- Christmas, Catholic Encyclopedia
The word "Christmas" is just a contraction of "Christ's
Mass" or "Mass of Christ", which was a special Catholic
Mass to celebrate Christ's birth.
I wonder if Christmas Eve communion services in
protestant churches today also stem from this practice?