http://en.wikipedia.org/wiki/Christmas_Day
It is unknown exactly when or why December 25 was originally associated with Jesus' birth; the New Testament does not name a specific date. [13] Sextus Julius Africanus popularized the idea that Jesus was born on December 25 in his Chronographiai, a reference book for Christians written in 221 AD.[13] This date is nine months after the traditional date of the Incarnation (March 25), now celebrated as the Feast of the Annunciation.[17] March 25 was also considered to be the date of the vernal equinox and therefore the creation of Adam.[17] Early Christians believed March 25 was also the date Jesus was crucified.[17] The Christian idea that Jesus was conceived on the same date that he died on the cross is consistent with a Jewish belief that a prophet lived an integral number of years.
The identification of the birthdate of Jesus did not at first inspire feasting or celebration. Tertullian does not mention it as a major feast day in the Church of Roman Africa. In 245, the theologian Origen denounced the idea of celebrating Jesus' birthday "as if he were a king pharaoh." He contended that only sinners, not saints, celebrated their birthdays.[7] His criticism may be inspired by his contact with Egypt as Origen was born in Alexandria.
The earliest reference to the celebration of Christmas is in the Calendar of Filocalus, an illuminated manuscript compiled in Rome in 354.[3][18] In the east, meanwhile, Christians celebrated the birth of Jesus as part of Epiphany (January 6), although this festival focused on the baptism of Jesus.