Dear Friends,
As an Armenian Christian, I would be remiss if I did not insist that Jan. 6th was the day of our Lord's Nativity.

Allow me to elaborate.
Leaving the issue of "calendars" aside (which is not standarized throughout the Armenian Church), the Armenian Church (to the best of my knowledge) is the only ancient Church which maintains the ancient dating of the Nativity on January 6th. Allow me to quote an Armenian text, "Saints and Sacraments of the Armenian Church," in which the author, Bishop Kaloustian writes,
"The reason for changing [the Nativity from January 6th to December 25th] is given [by a Roman Catholic] author:
'The Lord was born in the month of January on the same day on which we celebrate the Epiphany, for, of old, the feast of the Nativity and Epiphany were kept on one and the same day. The reason why our Fathers changed the solemnity celebrated on the sixth of January, and transfered it to the 25th of December is as follows: It was the custom of pagans to celebrate the birthday of the Sun on this very day, December 25th. In these solemnities and festival the Christians too were tempted to participate. The leaders of the Church, therefore, decided that the Birth of Christ, be kept on this date (i.e. 25th of December) to overshadow the pagan festival and on Januarty sixth, the feast of Epiphanies. (Catholic Encyclopedia: Articles, "Christmas" and "Epiphany").
Bishop Kaluostian continues:
"This change, having begun in Rome, spread very quickly to the East and all over the Roman Empire. The change did not penetrate into Armenia, because the Armenians, having no such pagan festival on December 25th to suppress, did not see any reason for following the new practice... the Armenians wanted to remain faithful to the ancient custom; so they have kept the old date throught the centuris to the present day." (pgs.62-63).
Thus on the 6th of January (whichever calendar they are using) Armenians celebrate the Nativity (Birth) and Theophany (Baptism) of Christ in one solemn Divne Liturgy as is the anceint tradition. Besides the conjecture of whether this was actually "the day" of Christ's Birth, I hope this helps to round out your information on the Eastern Churches dating and celebration of the Nativity of our Lord.
In Christ's Illuminating Light,
Wm. DerGhazarian
Kreesdos Dznav yev Haydnetsav!
(Christ is Born and Revealed!)
Orhnyal eh Haydnootyoonun Kreesdosee!
(Blessed is the Revelation of Christ!)