Today I saw someone tweet that "Christmas does NOT commemorate the INCARNATION of our Lord, but His NATIVITY." Seems relatively harmless, right? Here's another example of the same sentiment, rightly corrected by Fr. McNamara.
What's the problem? Simply put, the Feast of the Nativity of Christ (Christmas) is about the Incarnation. Fr. McNamara offers a pretty good, concise clarification. I'd like to elaborate a bit. First off, in the Orthodox tradition (see this and this, for example), the Nativity is inextricably related to the Incarnation. In the first reference, Archbishop Dmitri says "…the Nativity of Christ (December 25). Eight days later (January 1) we remembered the Circumcision of Christ and then His Baptism (January 6). The commemoration of these events in our Lord's earth [sic] life basically form one feast, the feast of the Incarnation of God the Word." In the second, His Grace Bishop Youssef uses "Feast of the Nativity" and "Feast of the Incarnation" interchangeably. I mention these because sometimes the Orthodox can give us a bit more insight into the ancient Christian faith than we get from only Roman Catholic sources.
Now back to Fr. McNamara, he notes that "the prayers used during Advent are taken from the ancient manuscripts known as the Scroll of Ravenna (fifth-sixth centuries) and the Gelasian sacramentary (seventh century). Their constant theme is the coming of Christ, both in the incarnation (first coming) and at the end of time (second coming)." He goes on to note that the Nativity was celebrated earlier than the Annunciation, and that these prayers that relate the Incarnation to Christmas are correspondingly more ancient.
It seems pretty clear to me that the chief meaning of the celebration of the Nativity is the Incarnation—the Word become flesh. It'd probably be helpful to distinguish between the basic, biological event of the incarnation and the Incarnation as a theological reality. Those who make the point in the tweet above are thinking (presumably) only of the former; the problem is that this is not the way that the Church traditionally thinks about it.
The feast of the Annunciation has its own particular emphasis on Mary's awesome and unique role in salvation history—her immaculateness and her fiat. That it follows this event would also be, biologically speaking, the beginning of Christ's life on earth seems to me to be of secondary concern for the Annunciation, and I think it is only recently that focusing on that aspect in relation to the Annunciation has become en vogue in Catholic circles due to pro-life concerns.
From a biological point of view (a pro-life-concern point of view), yes, it is the point in time when the Word took on human nature. But that's just not the point. You see, we know that the Word is eternal—from the beginning, and it is this Divine nature that makes the Incarnation so special. The Incarnation is first and foremost a theological event, not a biological one. This theological event of the Incarnation, you might say, is ultimately what the Annunciation all about. So if we're going to give the Annunciation celebration a secondary meaning, it should be the wonder at the Incarnation, not so much the fact that it was biologically the beginning of Jesus' human life.
Looking at it like this—with the Incarnation (as a theological event) as the central focus (and not the human conception of Jesus)—it's not hard to see that we can just as rightfully celebrate the Incarnation at Christmas. You see, the biological events (conception and birth) are not the important things here. There's nothing particularly special, unique, or wonderful about them in themselves—it is the Incarnation, the Word become flesh, that gives them both significance. And indeed, as Archbishop Dmitri points out, the other events in Jesus' earthly life are also occasions to celebrate the Incarnation. As events that situate Jesus as a real human being in time/history, they are important, but those are more like the background to the theological story. The Incarnation is not (just) about Jesus' human conception; it's about God becoming man so that man might become God (St. Athanasius). It's just that awesome.
So what bugs me about the tweet (apart from it being something of a novelty and historically ill-advised) is that it inadvertently obscures and diminishes the deeper, more important meaning and value of these two feasts. In a way, it subjugates the Incarnation to the pro-life cause, which is just wrong.
When speaking of the Incarnation, perhaps it'd more better to speak of the feasts of the Incarnation (instead of just one), or to speak of it, as Archbishop Dmitri did, as one mystical feast that spans many particular holy days (mirroring the same reality of Christ's Incarnation lived out across many days on earth). But in any case, it certainly is valid, good, and part of our ancient Tradition to celebrate the Incarnation during Advent and Christmas. Enjoy!