"Lraper.org: A question often asked is why Armenians do not celebrate Christmas on 25 December, as the rest of the world generally does. Just as chronologically there is no clear date for Christ's Holy Nativity, the Gospels also do not contain one. But historically all Christian Churches up until the fourth century celebrated the Festival of Christ's Nativity on 6 January.
According to the Roman Catholic Church, the date of 6 January was changed because the pagan traditional festival celebrated on 25 December that marked the birth of the Sun was declared invalid. But Christians continued to hold to those kinds of pagan festivals on that date. In order to break their influence, the Church hierarchy defined 25 December as Christmas, that is, as the Festival of the Holy Nativity of Christ, while 6 January was defined as the visit of the three magi to the newly born Christ.
Because the Armenians did not experience the problem of Saturnalia, i.e. the Festival of the Birth of the Sun, and because the Armenian Church was not a satellite of the Roman Church, Armenians were unaffected by this change.
According to church traditions, Armenians continue to celebrate Christmas on 6 January.
The Coptic Orthodox Church of Egypt carries on with the same tradition together with the Armenians. However, the Ethiopian and Russian Orthodox Churches treat 6 January as the eve of the festival, which is celebrated on 7 January."

I will add here that the Armenian Catholic Church celebrates Christmas on 25 Dec. following the traditions of the Roman Catholic Church. On the other hand, although the Evangelicals in the West celebrate on 25 Dec., the Armenian Evangelical Church celebrates Christmas on Jan. 6 (at least the Churches in the Near East).