There is only one day within the Christmas Octave (i.e. the 30th of December) which is not a Saint’s Feast. During the Octaves of the Epiphany, Easter and Pentecost, the Church is so absorbed in the respective mysteries that she puts off everything that could share her attention; whereas during this of Christmas, there is only one day which does not celebrate the memory of some glorious Saint, and our Infant Jesus is surrounded by a choir of heroes who loved and served him. Thus, the Church, or, more correctly, God—for God is the first author of the cycle of the year—shows us how the Incarnate Word, who came to save mankind, desires to give mankind confidence by this his adorable familiarity.

We have already shown that the Birth of our Lord took place on a Sunday, the Day on which, in the beginning of the world, God created Light. We shall find, later on, that his Resurrection also was on a Sunday. This the first day of creation, and the first of the week, was consecrated by the old Pagans to the Sun; with us Christians, it is most sacred and holy, on account of the two risings of our divine Sun of Justice—His Birth and His Resurrection. Whilst the solemnity of Easter is always kept on a Sunday, that of Christmas falls by turns on each of the days of the week—we have already had this difference explained to us by the Holy Fathers: but the mystery of Jesus’ Birth is more aptly and strongly expressed, when its anniversary falls on a Sunday. Other years, when the coincidence does not happen, the Faithful will at least be led by their Christian instincts, to give special honor to the day within the Octave which falls on the Sunday. The Church has honored it with a proper Mass and Office. . . .

The passage of the Gospel selected for this Mass, though bearing on the Divine Infancy, yet gives us, we may almost say prematurely, the terrible prophecy of Simeon regarding the dear Babe of Bethlehem. The heart of Mary, that was overflowing with joy at the miraculous Birth of her Child, is here made to feel the sword spoken of by the venerable Priest of the temple. Her Son, then, is to be but a sign that shall be contradicted! The mystery of man’s adoption by God is to cost this Child of hers his life! We that are the Redeemed in His Blood, we may not yet dwell on the fatigues and the Passion and the Death of our Emmanuel: the time will come for that. At Present we are forbidden to think of him other than as the sweet Child that is born to us, the source of all our happiness by his having come among us.

Commentary from The Liturgical Year, Dom Prosper Guéranger (1805-1875)