I may have posted this in the past, but just in case, here it is again. From Barbara G. Walker's The Woman's Encyclopedia of Myths and Secrets:
For its first three centuries, the Christian church knew no birthday for its savior. During the 4th century there was much argument about adoption of a date. Some favored the popular date of the Koreion, when the divine Virgin gave birth to the new Aeon in Alexandria.(1) Now called Twelfth Night or Epiphany, this date is still the official nativity in Armenian churches, and celebrated with more pomp than Christmas by the Greek Orthodox.(2)
Roman churchmen tended to favor the Mithraic winter-solstice festival called Dies Natalis Solis Invictus, Birthday of the Unconquered Sun.(3) Blended with the Greek sun-festival of the Helia by the emperor Aurelian, this December 25 nativity also honored such gods as Attis, Dionysus, Osiris, Syrian Baal, and other versions of the solar Son of Man who bore such titles as Light of the World, Sun of Righteousness, and Savior.(4) Most pagan Mysteries celebrated the birth of the Divine Child at the winter solstice. Norsemen celebrated the birthday of their Lord, Frey, at the nadir of the sun in the darkest days of winter, known to them as Yule. The night of birth, Christmas Eve, was called Modranect, Latin matrum noctem, the Night of the Mother - originally a greater festival than Christmas Day.(5)
Early in the 4th century the Roman church adopted December 25 because the people were used to calling it a god's birthday. But eastern churches refused to honor it until 375 A.D.(6) The fiction that some record existed in the land of Jesus' alleged birth certainly could not be upheld, for the church of Jerusalem continued to ignore the official date until the 7th century.(7)
Trappings such as Yule logs, gifts, lights, mistletoe, holly, carols, feasts, and processions were altogether pagan. They were drawn from worship of the Goddess as mother of the Divine Child. Christmas trees evolved from the pinea silva, pine groves attached to temples of the Great Mother. On the night before a holy day, Roman priests called dendrophori or "tree-bearers" cut one of the sacred pines, decorated it, and carried it into the tmeple to receive the effigy of Attis.(8) Figures and fetishes attached to such trees in latre centuries seem to have represented a whole pantheon of pagan dieties on the World Tree.
Christmas celebrations remained so obviously pagan over the years that manyh churchmen bitterly denounced their "carnal pomp and jollity." Polydor Virgil said: "Dancing, masques, mummeries, stage-plays, and other such Christmas disorders now in use with Christians, were derived from these Roman Saturnalian and Bacchanalian festivals; which should cause all pious Christians eternally to abominate them."(9) Puritans in 17th-century Massachusetts tried to ban Christmas altogether because of its overt heathenism.(10) Inevitably, the attempt failed. [Well hell yeah. Who doesn't like a good party with lots of good food, drink and gifts exchanged -- pagan roots or not?]
A curious mistake to the Christmas mystery play of the Towneley cycle shows a Great Mother image not fully assimilated to that of Mary. Before their attention was arrested by the annunciatory angel, idly chatting shepherds complained of their cruel overlords, and prayed "Our Lady" to curse them.(11) Considering that they were not acquainted with the Mother of Christ, a rather different "Lady" must have been intended.
Among many other superstitions connected with Christmas were some that were typical of pagan holy days, such as the belief that animals coulde speak human words at midnight on Christmas Eve, or that divinatory voices coulde be heard at crossroads at the same time.(12) Also at midingith on Christmas Eve, water in wells and springs was supposed to turn into blood, or its sacramental equivalent, wine. The miracle was not to be verified, however; for all who witnessed it would die within the year.(13)
(1) Campbell, M.I., 34.
(2) Miles, 22.
(3) Reinach, 282.
(4) H. Smith, 130; Hyde, 92; Miles, 23.
(5) Turville-Petre, 227.
(6) Frazer, G.B., 416.
(7) Miles, 22.
(8) Vermaerren, 115.
(9) Hazlit, 118-19.
(10) de Lys, 372.
(11) Miles, 135.
(12) Summers, V., 157.
(13) Miles, 234.