Holman Bible Dictionary 
In the early part of the fourth century, Christians in Rome began to celebrate the birth of Christ. The practice spread widely and rapidly, so that most parts of the Christian world observed the new festival by the end of the century. In the fourth century, the controversy over the nature of Christ, whether He was truly God or a created being, led to an increased emphasis on the doctrine of the incarnation, the affirmation that “the Word was made flesh” ( John 1:14 ). It is likely that the urgency to proclaim the incarnation was an important factor in the spread of the celebration of Christmas.
No evidence remains about the exact date of the birth of Christ. The December 25 date was chosen as much for practical reasons as for theological ones. Throughout the Roman Empire, various festivals were held in conjunction with the winter solstice. In Rome, the Feast of the Unconquerable Sun celebrated the beginning of the return of the sun. When Christianity became the religion of the Empire, the church either had to suppress the festivals or transform them. The winter solstice seemed an appropriate time to celebrate Christ's birth. Thus, the festival of the sun became a festival of the Son, the Light of the world. See Church Year .
Fred A. Grissom
Charles Buck Theological Dictionary 
The day on which the nativity of our blessed Saviour is celebrated. The first footsteps we find of the observation of this day are in the second century, about the time of the Emperor Commodus. The decretal epistles, indeed, carry it up a little higher, and say that Telesphorus, who lived in the reign of Antoninus Pius, ordered divine service to be celebrated, and an angelic hymn to be sung the night before the nativity of our Saviour. That it was kept before the time of Constantine we have a melancholy proof; for whilst the persecution raged under Dioclesian, who then kept his court at Nicomedia, that tyrant among other acts of cruelty, finding the multitudes of Christians assembled together to celebrate Christ's nativity, commanded the church doors where they were met to be shut, and fire to be put to it, which soon reduced them and the church to ashes.
Webster's Dictionary 
(n.) An annual church festival (December 25) and in some States a legal holiday, in memory of the birth of Christ, often celebrated by a particular church service, and also by special gifts, greetings, and hospitality.
Hastings' Dictionary of the New Testament 
Christmas —See Calendar, and Dates, § 1.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
We present the following additional particulars concerning this important festival:
"Pope Julius I confirmed the birthday of our Lord to be kept on December 25; and Chrysostom, in the 4th century, speaks of the feast as of great antiquity; Clement of Alexandria, in the beginning of the 3d century, speaks of it, but refers it to April 19 or 20, or May 20; and sermons of Basil and Gregory Nazianzen, preached on this day, are still extant. Epiphanins reckons it on January 6, but Augustine on December 25. From the West the observance of the day passed to the Eastern Church in the 4th century; as Chrysostom says, the feast was unknown at Antioch tell years before the time he was preaching, that is, probably, as kept on December 25, the day hitherto observed having been January 6. The Latins, and Africa, and the Greek Church, generally, however, held the Nativity on December 25, as appears from Jerome, Augustine, Chrysostom, Basil, and Gregory Nazianzel. The Orientals in Egypt, Cyprus, Antioch, and Palestine appear to have observed, for a time only, January 6, as the feast of. the Nativity and-Epiphany, or Theophania, name equally applicable to both, as Gregory Nazianzen observes.
However, about the beginning of the 5th century the Nativity was commemorated, in the East, on December 25, and the Epiphany on the later day. In the 6th century, beyond doubt, East and West agreed in their observance. The Basquecallit the New Day, because all things are become new — old things are passed away. Christmas Eve is called, in Celtic, the Night of Mary; in Germany, the Holy Night; in Portugal, the Pasch of the Nativity; and in old English, Yule Merriment. In the Isle of Man the peasants bring tapers to church, and sing carols; and in Germany they beat with mallets on the house door, to symbolize the anxiety of the spirits in prison to learn the glad tidings of the Nativity. There were three masses on this day: one at midnight on the eve [except in the Gallican, Mozarabic, and Armenian rites], commemorating the actual birth of our Lord; the second at dawn or cock-crow, its revelation to man in the shepherds; and the third at noon, the eternal sonship of the Holy Child Jesus. Two masses were said in France in the time of Gregory of Tours; but three masses were not introduced into Spain until the 14th century, nor at Milan until the 15th century. In the Medieval Church there was a representation of the shepherds, as at Lichfield, with a star gleaming in the chapel vault; and so lately as 1821 the Flemish preserved the same custom, and the peasants entering with sheep offered eggs and milk, while midnight mass was said at the high-altar. From the time of Augustine, midnight mass was said on the eve; and the Councils of Orleans and Toledo required all persons to attend this service at their cathedral church. The Christmas-box was a receptacle made of earthenware, in the 17th century, in which apprentices placed the rewards of their industry given them at that season."
The Nuttall Encyclopedia 
The festival in celebration of the birth of Christ now celebrated all over Christendom on 25th December, as coinciding with an old heathen festival celebrated at the winter solstice, the day of the return of the sun northward, and in jubilation of the prospect of the renewal of life in the spring.
- ↑ Christmas from Holman Bible Dictionary
- ↑ Christmas from Charles Buck Theological Dictionary
- ↑ Christmas from Webster's Dictionary
- ↑ Christmas from Hastings' Dictionary of the New Testament
- ↑ Christmas from Cyclopedia of Biblical, Theological and Ecclesiastical Literature
- ↑ Christmas from The Nuttall Encyclopedia