I've copied and pasted most of the following.
In a long section covering Luke 1:5 through 2:8, Luke writes of a specific series of events in chronological order. He begins by telling the story of Zacharias, a priest, and his wife Elizabeth, who were childless. While administering his priestly duties during the course of Abijah, Zacharias was visited by the angel Gabriel, who told him that his prayers had been answered and that he and Elizabeth would have a son. They were to name him John.
Because Zacharias doubted that this would happen, Gabriel informed him that he would not be able to speak until the birth of his son. As soon as his service in the Temple was completed, he returned to his own house. Elizabeth soon conceived and hid herself five months, unsure of how her pregnancy would be viewed.
In the sixth month of Elizabeth's pregnancy, Gabriel visited Mary and informed her, "And behold, you will conceive in your womb and bring forth a son, and shall call His name Jesus" (verse 31). Soon thereafter, Mary visited her cousin Elizabeth and stayed with her until the latter's ninth month, leaving just prior to John's birth. Jesus, then, was born approximately six months after John.
What information do we have up to this point?
» Zacharias, a priest, performed his duties during the course of Abijah.
» After he returned home from Jerusalem, Elizabeth conceived.
» Mary conceived in the sixth month of Elizabeth's pregnancy.
» John was born approximately six months before Jesus.
The Course of Abijah
To date Jesus' birth, we need a starting point. Fortunately, Luke supplies one in mentioning "the course of Abijah" (Luke 1:5). Is it possible to know if this course existed then, when it fell during the year, and how long it lasted?
Indeed it is!
I Chronicles 24 lists the courses, divisions or shifts of the priesthood that served in the Temple throughout the year. Verse 1 states, "These are the divisions of the sons of Aaron." Among the sons of Eleazar were sixteen heads of their father's house, while among the sons of Ithamar were eight additional heads of house, making twenty-four courses (verse 4).
These courses of priests were divided by lot to be officials of the sanctuary and of the house of God (verse 5). Beginning on Nisan 1, these courses rotated throughout the year, serving in the Temple for one week apiece. The course of Abijah, the course during which Zacharias was responsible to work, was the eighth shift (verse 10).
Josephus, the first-century Jewish historian—who was, by the way, of the priestly lineage of the course of Jehoiarib, the first course—supplies further information about the priestly courses.
"He [David] divided them also into courses: and when he had separated the priests from them, he found of these priests twenty-four courses, sixteen of the house of Eleazar and eight of that of Ithamar; and he ordained that one course should minister to God [during] eight days, from [noon] Sabbath to [noon on the following] Sabbath. And thus were the courses distributed by lot, in the presence of David, and Zadok and Abiathar the high priest, and of all the rulers: and that course which came up first was written down as the first, and accordingly the second, and so on to the twenty-fourth; and this partition hath remained to this day" (Antiquities of the Jews, 7:14.7).
These courses were strictly followed until the Temple was destroyed in ad 70.
The Talmud describes the details of the rotation of courses, beginning on Nisan 1. With only twenty-four courses, obviously each course was required to work twice a year, leaving three extra weeks. (The Hebrew year normally has fifty-one weeks. Intercalary, or leap, years have an additional four weeks.) The three holy day seasons, Passover, Pentecost and Tabernacles, during which all the courses were required to serve, made up these three extra weeks. Thus, each of the courses worked five weeks out of the year: two in their specific courses and three during the holy day seasons.
John the Baptist was sent to prepare the way for Messiah (Malachi 3:1; Luke 1:13-17). The gospel accounts make it very clear that he was born about half a year before Jesus was born. From historical details in Luke's account especially, as well as the accuracy of the Seventy Weeks prophecy (see "Seventy Weeks Are Determined . . .," p. 2), it is clear that Jesus was born sometime in 4 bc. This means, counting back the nine months of gestation and the six-month difference in age, John must have been conceived in the first half of 5 bc.
This fact forces us to choose the first shift of the course of Abijah as the time when Gabriel visited Zacharias in the Temple. Frederick R. Coulter, in his A Harmony of the Gospels (p. 9), computes it this way:
In the year 5 bc, the first day of the first month, the month of Nisan, according to the Hebrew Calendar, was a Sabbath. According to computer calculation synchronizing the Hebrew Calendar and the stylized Julian Calendar, it was April 8. Projecting forward, the assignments course by course, and week by week, were: Course 1, the first week; Course 2, the second week; all Courses for the Passover and Feast of Unleavened Bread, the third week; Course 3, the fourth week; Course 4, the fifth week; Course 5, the sixth week; Course 6, the seventh week; Course 7, the eighth week; Course 8, the ninth week; and all courses [sic] the tenth week, which was the week of Pentecost.
Zacharias of the course of Abijah worked the ninth week in his assigned course and the tenth week in the Pentecost course, and this period ran from Iyar 27 through Sivan 12 (Hebrew calendar) or June 3 through 17 (Julian calendar). He probably returned home immediately after his shifts were completed, and Elizabeth most likely conceived in the following two-week period, June 18 through July 1, 5 BC.
With this information we can calculate Elizabeth's sixth month as December, during which Mary also conceived (Luke 1:26-38). It is probable, because of the circumstances shown in Luke 1, that Mary conceived during the last two weeks of Elizabeth's sixth month. Thus, John was born in the spring of 4 BC, probably between March 18 and 31. By projecting forward another six months to Jesus' birth, the most probable time for His birth occurred between September 16 and 29. It is an interesting sidelight that Tishri 1, the Feast of Trumpets, is one of the two middle days of this time period.
I copied the above from a website that has a different purpose than I do. They want to toss out Dec.25, as it's obviously not the date of Jesus' birth.(ttp://sabbath.org/index.cfm/fuseaction/Library.sr/CT/ARTB/k/568/subj/christmas/When-Was-Jesus-Born.htm)
I, however, find it absolutely fascinating that Jesus was CONCEIVED around Dec 25, and became Immanuel, God With Us.
Anyone still wondering when life begins?
I've known for decades that this time of year has many pagan holidays, but I celebrated the birth of Christ then. Now the residual guilt at celebrating during pagan holidays is all gone, because Christians are celebrating Jesus' conception. We just move back Bethlehem, shepherds, hosts of angels,etc., about 9 months.