As a major solemnity Christmas generally plays a counterpoint to Easter: birth and re-birth, I suppose. It also has special relations to two other major solemnities -- Epiphany and Annunciation, and that's harder to pin down. Annunciation is the Feast of Incarnation; Epiphany is the Feast of Manifestation; but Christmas is a little of both, and has tended liturgically to absorb all their features. The Annunciation is about the Incarnation coming through faith; the Epiphany about having what was promised in full public sight; Christmas is a sort of odd twilight in between. We don't usually have much in the way of special celebrations of the Annunciation on March 25, and what we do have tends to celebrate it as a Marian feast, i.e., as Lady Day; there are still places and cultures where Epiphany is a big deal, but for the most part everything associated with it has over the years been swallowed up by Christmas. One reason seems to be liturgical -- we hang the movable feasts on the only real candidate for that, Easter, and we need another major feast to serve as the peg on which we hang the immovable ones. Annunciation and Christmas are the most plausible candidates, and Christmas has the advantage of being farther removed from Easter, and so capable of marking out the difference more clearly. Thus the role of Advent in the liturgical year, which, when diagrammed, becomes very clear.
But perhaps the chief reason is the power of its image. Easter has a powerful image: the Risen Christ in whom we are raised. But the image for the Feast of the Annunciation is very subtle. As Oscar Wilde wrote of it:
Was this His coming! I had hoped to see
A scene of wondrous glory, as was told
Of some great God who in a rain of gold
Broke open bars and fell on Danaë,
Or a dread vision as when Semele,
Sickening for love and unappeased desire,
Prayed to see God's clear body, and the fire
Caught her brown limbs and slew her utterly.
With such glad dreams I sought this holy place
And now with wondering eyes and heart I stand
Before this supreme mystery of Love:
Some kneeling girl with passionless pale face,
An angel with a lily in his hand
And over both the white wings of a dove.
Certainly lovely, but not overwhelming -- not a shower of gold, nor a devouring flame. Something much more indirect. Epiphany has its Star, but that's also indirect, and what it has beyond that it shares with Christmas. Christmas dominates over all, to some extent overbalancing even Easter, because it gives an image that simply blows the mind. We owe it to Luke the evangelist, who tells us of Mary and Joseph in Bethlehem:
While they were there, the time came for her to have her child, and she gave birth to her firstborn son. She wrapped him in swaddling clothes and laid him in a manger, because there was no room for them in the inn.
And again tells us of the message of the angels to the shepherds:
The angel said to them, "Do not be afraid; for behold, I proclaim to you good news of great joy that will be for all the people. For today in the city of David a savior has been born for you who is Messiah and Lord. And this will be a sign for you: you will find an infant wrapped in swaddling clothes and lying in a manger."
And there we have it, thrown out almost casually, if angels can throw out anything almost casually. There is good news of great joy, and it is this: a savior has been born who is Messiah and Lord. And the sign of it is a blanket-wrapped baby in a feeding trough. As Gregory rightly puts it in the sermon linked to above:
But what shall I say and what declare? I see the carpenter and the manger, the Infant and the Virgin Birth-Giver, forsaken by all, weighed down by hardship and want. Behold, to what a degree of humiliation the great God hath descended. For our sakes "impoverished, Who was rich" (2 Cor 8:9): He was put into but sorry swaddling cloths -- not on a soft bed. O poverty, source of all exaltation! O destitution, revealing all treasures! He doth appear to the poor -- and the poor He maketh rich; He doth lay in an animal manger -- and by His word He sets in motion all the world. He is wrapped in tattered swaddling cloths -- and shatters the bonds of sinners having called the entire world into being by His Word alone.
And so we see the significance of Christmas. Annunciation is the Feast of the Incarnation, the Word made flesh; Epiphany is the Feast of His manifestation to the world as flesh. But Christmas grabs us, seizes us, because it is the Feast of His Humility, that he did not regard equality with God something to hold tight, but emptied himself, taking the form of a servant, coming in human likeness, found in human appearance, humbling himself. God wrapped in a blanket, lying in a trough in some cave in a tiny little town because no one had room for him elsewhere; unheralded and unsung except by angels in the heavens and shepherds coming in from the fields. Luke knew what he was doing; what he wrote down was one of the most memorable religious images in all of history. It seizes the mind, overwhelms it, sets it alight, and moves it to action.
This is why, incidentally, I am always wary about Christians criticizing people for celebrating Christmas, which we often do. We should not do this -- no, not if they put up their lights and tree ten weeks early, nor if they listen to inane songs, or whatever other random fits of daffing with which they may go crazy. They are caught in the grip of an image that cannot be shaken; it inflames them with a fever that they can hardly bear. It grabs their hearts by their handles and pours them out until they are half-mad and all irritable from the strain of it. And, absurd as some of the festivities may be, the fire that lights them is a little bit contagious; even on the fringes, where people isolate themselves as much as possible from the religious side, one still feels its influences. It's not always the healthiest madness, but it is a forgivable one. When Dionysius descends, can people not be caught up in the bacchanal? How can they hold back, and not romp in reverence? Is there nothing to be enthusiastic about in the celebration of God's own humility, rich in giving, unashamed to be poor? By one gift beyond all expectation, we are inspired in our own myriad little ways, however faulty, however absurd. But if God was so humble that he did not shirk being a poor baby boy laid in a trough, we are called to the same humility; to humble ourselves in giving, and, failing that, to humble ourselves to those things others do that we deem foolish or absurd or tacky. God had to put up with your own folly and absurdity and tackiness, but He did not hesitate to endure it, and, more than endure it, associate Himself with it, if that's what it took to bring you to light. And that's what it took. We should all let the humility rub off on us a bit.
By the way, if you haven't seen it, you should check out The Logic Museum's page on the Vulgate version of Luke's Nativity story. You can see a Latin manuscript, its transcription, and the English translation, all beautifully done.