The Season of Christmas

Dom Guéranger writes in The Liturgical Year:

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, 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.

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 our 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 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 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.

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.