Why did Christians celebrate the Birth of Jesus on the 25th of December?
In my This places the dating of Christmas well before Theodosius made Christianity the religion of the Roman Empire (380), before Constantine made Christianity a legal religion in the Roman Empire (312), and also before Sol Invictus became the Pagan holiday on December 25th (274). This of course leads to the natural question, where did early Christians get the idea of celebrating the birth of Jesus on December 25th. I talked about how Hippolytus of Rome (around the year 200) took for granted that Jesus was born on the 25th of December.
I believe the answer to such a question can be gleaned from the New Testament. The Day of Atonement/ Yom Kippur takes place on the 10th day of the month of Tishrei, which is nine days after Rosh Hashanah (the Jewish New Year). The joys of working with a Jewish calendar versus the Julian or Gregorian calendars is that there is not a one to one correspondence. This means that, according to the calendars of the goyim, Yom Kippur falls somewhere between middle September to middle October. All of this is relevant because of some dating in Luke’s Gospel.
If we take that Yom Kippur happens sometime around the beginning of October or late September, and we add that the priest only entered the Holy of Holies on Yom Kippur, then we can do simple math to figure out the date when Jesus was born.
In Hebrews 9:7, we are told about the Holy of Holies that “only the high priest goes, and he but once a year.” This means that only once a year would a priest enter the Holy of Holies to burn incense and sprinkle blood on the altar inside the Holy of Holies.
Turning to Luke, we find that Zechariah was “was chosen by lot to enter the temple of the Lord and burn incense” (Luke 1:9). Luke goes on to tell how “the whole multitude of the people were praying outside at the hour of incense.” (Luke 1:10). At this moment, Zechariah saw the angel of the Lord standing on the right of the altar of incense. The angel announced that Zechariah and Elizabeth would have a son in their old age. This all happened between the middle of September and the middle of October.
Then we arrive at the important (for the purpose of their inquiry) statement in Luke 1:23–24:
“And when his time of service was ended, he went to his home. After these days his wife Elizabeth conceived.” This is important because Elizabeth conceived after Yom Kippur. But how soon? This is the weakest part of the argument. I think that the best understanding for the phrase “after these days” is that the conception of John the Baptist took place directly after Zechariah’s time of service. If this is the case, then we can plot out the timeline fairly easily.
“Elizabeth conceived, and for five months she kept herself hidden, saying, "Thus the Lord has done for me in the days when he looked on me, to take away my reproach among people." In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin's name was Mary.” (Luke 1:24–27)
Following the timeline, Elizabeth was six months pregnant when Mary conceived Jesus. Now for the math:
John the Baptist was conceived somewhere between middle September to the middle of October (tradition puts this on the 23rd of September).
Six months later, Mary conceived sometime in March or April (tradition puts it at March 25th)
Nine months after March or April puts us into late December or early January.
This is terribly imprecise, but it shows that December 25th is a biological and mathematical possibility for the birth of Jesus. Thus when taken together with early Christian attestations for the birth of Christ being on December 25th, the onus is put squarely on those who argue for a date other than December 25th for the birth of Jesus. It is an argument against Scripture and Tradition.
 See also: Leviticus 16:12–13 “And he shall take a censer full of coals of fire from the altar before the LORD, and two handfuls of sweet incense beaten small, and he shall bring it inside the veil and put the incense on the fire before the LORD, that the cloud of the incense may cover the mercy seat that is over the testimony, so that he does not die.”