One important point to understand the train of thought on Christmas December 25 is Gabriel’s visit to Mary in sixth month, as recounted in Luke 1:26-27.
And in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
“Sixth month” in Luke 1:26 internally refers to Luke 1:36, saying:
And, behold, your cousin Elizabeth, she has also conceived a son in her old age: and this is the sixth month with her, who was called barren. (Luke 1: 36)
This verse indicates that “sixth month” is relative to the beginning of Elizabeth’s pregnancy, 17th of Tishri 3756. And, the date of Gabriel’s visit to Mary is as follow:
||Julian Calendar||Gregorian Calendar
|17th of Nisan 3756||March 25th, 5 BC||March 23rd, 5 BC|
In conclusion, Gabriel visited Mary on 17th of Nisan 3756 Hebrew Calendar or March 25th, 5 AD Julian Calendar.