Where and how did Christmas start?
Christmas or "Christ's Mass," the celebration of Jesus Christ's birth, began in European and Middle Eastern Christendom in the 3rd Century. These early celebrations, feasts, or "Masses" were not standardized or widely held. It is thought that the early Christians did not widely celebrate the birth of Jesus. This is most likely true since the primary focus was on His life, crucifixion, and particularly His resurrection (Easter).
There certainly were some early celebrations. Early writings including a "feast calendar" written in 243AD indicate that there were some celebrations in the third century and perhaps in the second century. Christmas celebrations did not gain widespread prominence however until the Middle Ages or starting from around 400AD.
These celebrations began and occurred in the areas where Christianity started and spread, the areas around the Mediterranean, including the Middle East, North Africa, southern Europe.
Christmas day, December 25, is not necessarily the actual date of Christ's birth. The true date may not be known because we lack enough information to pinpoint it precisely. (Beware that there are many who claim to have calculated the actual date and are most likely no more accurate than December 25.) Other dates including December 25 were used for this feast.
December 25 was settled on by the early church for reasons that are not absolutely clear (but there are some logical reasons for that date). There were pagan celebrations on and around December 25th. There were pagan holidays year round.
No one can be absolutely certain of the exact day of Christ's birth. An early winter date is as reasonable a guess as any and December 25th has been the frontrunner for eighteen centuries.
There are many theories surrounding Christmas, such as the belief that it was created simply to convert pagans and/or replace the pagan celebrations around the winter solstice. The problem with these theories is two-fold. First, there is a lack of evidence. There was no big push in early church to create a birthday celebration for Christ. Of course, the church was and is "in the business" of spreading Christianity so there is no doubt that the early church fathers wanted pagans to convert and wanted celebrations to honor God.
Second, pagan worship practices were seasonal and on-going. That is, almost any date or time frame could be said to be a pagan "Holy" day. That Christian Holy Days would clash with and even replace pagan days was inevitable.
In short, Christmas began in the early areas of Christendom in the 2nd or 3rd Centuries.
It is also Jesus Christ's Birthday
Actually it is possible to approximately determine when Jesus' birth was. First of all the Gospel of Matthew that Jesus' birth occurred before the death of Herod the Great (Matthew 2:15), which occurred in the spring of 4 BC (or BCE) so Jesus's birth most likely occurred in 5 BCE. secondly the Gospel of Luke records that Zachariah, John the Baptist's father, was serving in the Temple as a priest of the Division of Abijah, which was the 8th of 24 priestly divisions. This service would have ended around May 20th 6 BC. This would have resulted in a likely birth of John the Baptist around the end of February 5 BC, and Jesus birth would have likely occurred near the end of August or the first week of September 5 BC. In any case Jesus was not born on Dec. 25. That date was chosen for Christmas because it was the date when the Winter Solstice occurred. THis date was widely celebrated as the birth date of the Persian sun god Mithras, who was widely worshipped in the Roman Empire, particularly among soldiers. The Emperor Constantine was among those who worshiped Mithras. When the chucrch became flooded with pagan converts, a huge problem arose. The converted pagans did not want to give up the Winter Solstice festival as it was a time of much celebrating. The church basically followed the old saying "If you can't beat them, join them" and decreed that December 25 should not be celebrated as the birth date of the sun god, but rather as the birth date of the Son of God. Christmas is a pagan holiday and the fact that virtually all of the popular customs observed on this date are of pagan origin confirms that fact.