From The Liturgical Year: Vol 2 - Book 1. By Dom Prosper Gueranger, O.S.B.
We apply the name 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, as distinct, by its own special spirit, from every other, as are Advent, Lent, Easter, or Pentecost. One same Mystery is celebrated and kept in view during the whole forty days. Neither the Feasts of the Saints, which so abound during this Season; nor the time of Septuagesima,
with its mournful Purple, which often begins before Christmastide is over, seem 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. The Faithful will remember that the Liturgy commemorates this long expectation by the four penitential weeks of Advent.
The custom of celebrating the Solemnity of our Saviour's Nativity by a feast or commemoration of forty days' duration is founded on the Holy Gospel itself; for it tells us that the Blessed Virgin Mary, after spending forty days in the contemplation of the Divine Fruit of her glorious Maternity, went to the Temple, there to fulfill, in most perfect humility,the ceremonies which the Law demanded of the daughters of Israel, when they became mothers.
A ROMAN FEAST
The Feast of Mary's Purification is, therefore, part of that of Jesus' Birth; and the custom of keeping this holy and glorious period of forty days as one continued Festival has every appearance of being a very ancient one, at least in the Roman Church. And firstly, with regard to ou Saviour's Birth on December 25, we have St John Chrysostom 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 Saviour's Birth, since the acts of the Enrollment, taken in Judaea 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 Saviour 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.
CHRISTMAS and EPIPHANY
But it was not till the fourth century that the Churches of the East began to keep the Feast of our Saviour's Birth in the month of December. Up to that period they had kept it at one time on the sixth of January, thus uniting it, under the generic term of Epiphany, with the Manifestation of our Saviour made to the Magi, and in them to the Gentiles; at another time, as St Clement of Alexandria tells us, they kept it on the 25th of the month Pachon (May 15) , or on the 25th of the month Pharmuth (April 20). St John
Chrysostom, in the Homily we have just cited, which he gave in 386, tells us that the Roman custom of celebrating the Birth of our Saviour on December 25 had then only been observed ten years in the Church of Antioch.
It is probable that this change had been introduced in obedience to the wishes of the Apostolic See, wishes which received additional weight by the edict of the Emperors Theodosius and Valentinian, which appeared towards the close of the fourth century, and decreed that the Nativity and Epiphany of our Lord should be made two distinct Festivals.
The only Church that has maintained the custom of celebrating the two mysteries on January 6 is that of Armenia; owing, no doubt, to the circumstance of that country not being under the authority of the Emperors; as also because it was withdrawn at an early period from the influence of Rome by schism and heresy. [.]
AN INFANT-GOD and a VIRGIN-MOTHER
But what is the characteristic of Christmas in the Latin Liturgy? It is twofold: it is joy, which the whole Church feels at the coming of the Divine Word in the Flesh; and it is admiration of that glorious Virgin, who was made the Mother of God. There is scarcely a prayer, or a rite, in the Liturgy of this glad Season, which does not imply these two grand Mysteries: an Infant-God, and a Virgin-Mother.[.]
The Liturgy never loses sight of the Divine Babe and his incomparable Mother, and never tires in their praises, during the whole period from the Nativity to the day when Mary comes to the Temple to present her Jesus.
The Greeks, too, make frequent commemorations of the Maternity of Mary in their Offices of this Season: but they have a special veneration for the twelve days between Christmas Day and the Epiphany, which, in their Liturgy, are called the Dodecameron. During this time they observe no days of Abstinence from flesh-meat; and the Emperors of the East had, out of respect for the great Mystery, decreed that no servile work should be done, and that the Courts of Law should be closed, until after January 6.
From this outline of the history of the holy season, we can understand what is the characteristic of this second portion of the Liturgical Year, which we call Christmas, and which has ever been a season most dear to the Christian world.