I wonder why the Apostles never celebrated Christ’s birth? And if they did, I wonder why such celebrations are not in the Bible?
John says at the end of his Gospel that there is much more that could have been recorded that was not recorded. Just because the Bible does not record the celebration of Jesus' birth doesn't mean such celebrations did not happen. It is not necessary to know the actual calendar date of Jesus' birth to believe in him.
The Apostles celebrated His Resurrection. That is far more important than His birth. IIRC, birthdays were not considered important events in those times. Descendancy was what mattered and still does, in that part of the world as evidenced by Matthew 1:1-17. The first question asked, even today, is "What is your (family) name?"