It is not stated if and when it was celebrated. Then again, the ending of John states that all the things that transpired regarding Christ are not contained in the Gospels and that it would take many books to record all He said and did. As for the date, I have seen a number of estimates with some falling in March while an article some years back pointed to a date near late December. It is a small point as it is an anniversary, isn't it? It IS celebrated since it marks the point at which the Word became Flesh (manifestly as an infant) marking the Divine becoming Human and thereby recognizable to all outside His mother's womb as God-Man.
But again, the only reason I brought it up was to respond to the other poster. I personally find Christ Mass to be a beautiful holiday and I love the music and celebrations that go with it. I don't participate personally but I do that out of respect because I am not a christian. ;)