Not including Jesus when did Christmas start to be celebrated?

The celebration of December 25 originated with the religious and mythological beliefs of the Romans. The day was called Saturnalia. Saturn was the Sun God or God of the sky. The day was to celebrate the end of winter where daylight was shorter and the beginning of spring where the daylight starts to extend. To them it was the rebirth of the sun. They celebrated and worshipped on the day to their god Saturn. During the 3rd and 4th centuries as Christianity began to have its upsurge in Rome - hence the term Roman Catholic - efforts began to be made to Christianize the dominion of Rome which was at the time most of the known world. Many expressed strong opposition to Christianity which led to the so-called Holy Wars. During which time many groups were subjugated to become Christians and ones that resisted were tortured and/or killed. In order to make the transition from a pagan religion to Christianity easy for these pagan groups, the clergy took their pagan celebrations and converted them to Christian celebrations by removing their pagan god and installing Christ. Saturnalia then became the celebration of the birth of Christ instead of the birth of the Sun and a day to Christ instead of a day to Saturn. It was an easy change for many to accept though since the change was only nominal and the traditions observed associated with day remained the same. Christmas is not a Christian celebration and would have been condemned in the 1st two centuries as shown in the clip from the Encyclopedia Brittanica below. From the Enclopedia Brittanica: "During the first two centuries of Christianity there was strong opposition to recognizing birthdays of martyrs or, for that matter, of Jesus. Numerous church fathers offered sarcastic comments about the pagan custom of celebrating birthdays when, in fact, saints and martyrs should be honoured on the days of their martyrdom....The precise origin of assigning December 25 as the birth date of Jesus is unclear. The New Testament provides no clues in this regard. December 25 was first identified as the date of Jesus' birth by Sextus Julius Africanus in 221 [In the 3rd century A.D. nearly 180 years after the death of Jesus) and later became the universally accepted date. One widespread explanation of the origin of this date is that December 25 was the Christianizing of the dies solis invicti nati ("day of the birth of the unconquered sun"), a popular holiday in the Roman Empire that celebrated the winter solstice as a symbol of the resurgence of the sun, the casting away of winter and the heralding of the rebirth of spring and summer. Indeed, after December 25 had become widely accepted as the date of Jesus' birth, Christian writers frequently made the connection between the rebirth of the sun and the birth of the Son. One of the difficulties with this view is that it suggests a nonchalant willingness on the part of the Christian church to appropriate a pagan festival when the early church was so intent on distinguishing itself categorically from pagan beliefs and practices."