All Orthodox Churches observe the Nativity on December 25th. The question is "when is December 25th?" As Kolokotronis mentions, there are two Calendars in use in the Orthodox Church. The Old (Julian) calendar is currently 13 days behind the New (Western) calendar, so when it is the 25th of December to a church on the Old Calendar, the civil calendar reads January 7.
Worldwide, the vast majority of Orthodox Christians follow the Old Calendar, because of the large populations of Russia, Serbia, the Ukraine, Georgia, and other Slavic countries. The Jerusalem Patriarchate also follows the Old Calendar, as do many Greek monasteries -- all of the monasteries on Mt. Athos, the monastery at Mt. Sinai, the monastery on Patmos, etc... all remain on the Old Calendar. There are sizable Old Calendar populations in Romania, and to a lesser extent, in Greece.
Here in America, most are on the New, because the majority of Orthodox Christians here are Greek. The OCA changed to the New Calendar in the 1980's except for the Diocese of Alaska, which changed only a few years ago. The Serbs, the ROCOR, the Moscow Patriarchate parishes, most Ukrainian Orthodox, most non-OCA Carpatho Russians, and scattered OCA parishes are all on the Old.
Regarding the matter of Liturgy if Annunciation falls on Holy Friday, the rubrics in the Orthodox Church are as follows:
On the eve of the feast, (Holy Thursday), Great Vespers are sung, followed by the Liturgy of St. Basil (this would usually be done in the late morning or mid-day. Great Compline is not said. Matins of the feast are combined with the service of the 12 Gospels (the usual Holy Friday service that is served on the eve). On the day of the feast (i.e. Holy Friday itself), Great Vespers followed by the Liturgy of St. John Chrysostom.
So, the rule of "no liturgy on Holy Friday" (which we Orthodox also have) has been specifically given one exception in the Orthodox Church from time immemorial -- Annunciation. The serving of Divine Liturgy on the feast of Annunciation is never displaced, not even by Holy Friday.
This should not be surprising, since in the Orthodox Church, the greatest emphasis has always been on Christ's life and Resurrection. If you attend our lengthy daily Holy Week services, you would see that we certainly do not at all give short shrift to the significance of Christ's suffering and death, but even there, if you listen to the hymns, they are laced with life and foreshadowings of the Resurrection.
This moment at which God became man, this moment in which the Creator entered his creation and in his human nature became subject to time and space (which is what happened at the Annunciation -- he was conceived in the womb of the Theotokos) changed the entire universe. The particular way in which official human thuggery killed Jesus Christ can in no way be seen as being more important than the divine condescension of His becoming man in the first place. Thus, we celebrate the Liturgy on Holy Friday if Annunciation falls on it. In a sense, it almost supercedes Holy Friday liturgically -- at the very least, it stands beside it as being of equal importance. Even when it falls on Pascha itself, it is not displaced, and the services are combined.
"I believe that the Armenians (who are Eastern, but not Orthodox) may observe the Nativity on January 6th rather than Theophany, but this is a bit fuzzy in my memory."
Your memory isn't as fuzzy as you thought. The Armenians are the only ones, and it is an ancient, ancient tradition.
Your comments on the possible convergence of the Feast of the Annunciation and Great Friday point up a major mindset difference between the Church in the West and the Church in the East. While the Passion surely is important in Orthodoxy, nevertheless, the Incarnation which allows Man to become divinized as God took on human form, and the descent into Hell and Resurrection whereby the power of death and the Evil One were overthrown as so magnificently expressed by St. John Chrysostomos speak to a rather different concept of salvation in the East from that in the West.
Pardon my confusion... Since the article about "March 25th" was posted on the Western March 25th, I was discounting the calendar shift.
Also, the Eastern religionists I presently work with are Armenian, furthering my confusion. (I should know better... My confirmation sponsor was Byzantine!)
Pardon my confusion... Since the article about "March 25th" was posted on the Western March 25th, I was discounting the calendar shift.
Also, the Eastern religionists I presently work with are Armenian, furthering my confusion. (I should know better... My confirmation sponsor was Byzantine!)