"We apply the name of Christmas to the forty days which begin with the Nativity of our Lord, December 25, and end with the Purification of the Blessed Virgin, February 2. It is a period which forms a distinct portion of the Liturgical Year..." He adds that nothing is "able to distract our Holy Mother the Church from the immense joy of which she received the good tidings from the Angels on that glorious Night for which the world had been longing four thousand years."
With regard to our Savior's Birth on December 25, we have St. John Chrysostom (d.407A.D.) telling us, in his Homily for this Feast, that the Western Churches had, from the very commencement of Christianity, kept it on this day. He is not satisfied with merely mentioning the tradition; he undertakes to show that it is well founded, inasmuch as the Church of Rome had every means of knowing the true day of our Savior's Birth, since the acts of the Enrolment, taken in Judea by command of Augustus, were kept in the public archives of Rome. The holy Doctor adduces a second argument, which he founds upon the Gospel of St. Luke, and he reasons thus: we know from the sacred Scriptures that it must have been in the fast of the seventh month that the priest Zachary had the vision in the Temple; after which Elizabeth, his wife, conceived St. John the Baptist: hence it follows that the Blessed Virgin Mary having, as the Evangelist St. Luke relates, received the Angel Gabriel's visit, and conceived the Savior of the world in the sixth month of Elizabeth's pregnancy, that is to say, in March, the Birth of Jesus must have taken place in the month of December."
* I just found this online. Merry Christmas Christian Catholics
Bump for Christmas time dates!
Origin of the Twelve Days of Christmas [An Underground Catechism]