The date of birth of Christ was the object of speculation from the early 3rd century, various churchmen suggesting different dates. However, the celebration of the anniversary of Christ's birth does not appear to have become widespread until the 4th century. The earliest mention of observing the Nativity of our Lord on December 25 is in a calendar tabulating the practice in Rome in the year 336. According to some scholars, this date was perhaps chosen to counteract the pagan feast of the nativity of the Invincible Sun, Christ being denoted as the 'Sun of Righteousness'. From Rome the celebration of the Nativity spread to the rest of the West.
In the East it was the feast of the Epiphany on January 6 that primarily commemorated the Baptism of Christ. But in the later part of the 4th century, Epiphany Day was connected with the Nativity, especially in Syria and by the middle of the 5th century most of the Eastern Churches had adopted December 25 as a separate feast of our Lords' Nativity. However, the Church of Jerusalem celebrated on January 6 both Nativity and Baptism until 549, and in the Armenian Church January 6 is still observed as Christmas Day.
The feast of the Nativity has been popularly observed in the West with joy and merry -making both characteristics of the Roman festival of the Sun and other pagan feasts it replaced. Today's Christmas tree, adopted in recent years by the Orthodox in many parts of Europe and America, was imported to England in the 19th century by Albert, the Prince Consort, together with other German customs.
from A Dictionary of Greek
Hellenic Heritage Publications