Caesar Augustus became the ruler of the Roman Empire in 27 B.C. and ruled until A.D. 4 when he died. This does not narrow the birth of time of Jesus much, but gives important information.
Luke 1:5-2 explains, "There was in the days of Herod, the king of Judaea, a certain priest named Zacharias, of the course of Abia: and his wife was of the daughters of Aaron, and her name was Elisabeth. And they were both righteous before God, walking in all the commandments and ordinances of the Lord blameless." (Luke 1:5-6)
Luke 1:24 gives tells us that the Elizabeth, the mother of John the Baptist conceived six months before the Holy Spirit conceived Jesus in Mary. "And after those days his wife Elisabeth conceived, and hid herself five months, saying," (Luke 1:24) This is extremely important in determining the birth of Christ.
Zacharias, Elizabethís husband was a priest and served in the temple during the "course" of Abia. (ah - bib or a bi ah) This gives us a way to narrow down the birth of Jesus. David divided the priests and Levites into 24 groups of priests and this set the schedule for when the individual priests would serve in the Temple. ( 1 Chron. 28:11-13,19) They then due lots to see when each group of priests to determine when eacj priest would serve in the Temple. (Neh. 10:34)
Luke 1:5 says "There was in the days of Herod, the king of Judaea, a certain priest named Zacharias, of the course of Abia: and his wife was of the daughters of Aaron, and her name was Elisabeth." The phrase "course of Abia" refers to the group of priests to which Zacharias whom served in the Temple in the month of Sivan (May-June). Historical calculation would put his service in June 13-19.
A priest would serve for one week beginning on Saturday the Sabbath. Zacharias probably completed his Temple service the third week of Sivan. Afterward he returned home Elizabeth conceived John the Baptist. This would date around the middle to the end of June.
Luke 1:24-27 provided the important detail that Jesus was conceived of the Holy Spirit six months after Elizabeth, Maryís cousin, conceived.
Six months (verse 26) would mean that Jesus was conceived in the Jewish month of Kislev (kiss-leav - Nov-Dec). Hanukkah, called the Feast or Festival of Lights was on Kislev 25th. We cannot be dogmatic about this, but that would seem to be the perfect time for Christ to be conceived as He came as the Light unto the world.
"Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life." (John 8:12)
As Elizabeth conceived Johnís near the end of June, adding nine months brings us to the end of March as the most likely time for Johnís birth. Adding another six months (the difference in ages between John and Jesus according to Luke 1:35-36) this brings us to the end of September as the likely time of Jesusí birth.
Jesus was probably born on 15h of Tishrei (tris he dee - in Hebrew) or (Tris he ray) which was the end of September. Tishrei is the first month of the Jewish year or Ras Hashannah. Yom Kippur , the Day of Atonement, is the 10th day and Sukkot (Su cous) is the 15th Day of the first month called the Feast of Tabernacles. It celebrates the restoration of Israelís relationship with the Lord after 40 year of wandering in the desert. Sukkot is a time of rejoicing and spiritually corresponds to the joy of knowing your sins are forgiven (during Yom Kippur. It further looks to the coming of the Messiah.
Therefore Jesus was clearly not born on December 25th, but near the end of the month of September. If born on Tishrei 15th, it is difficult to pen point the day on the Gregorian calendar month of September, because the Jewish calendar is based on the lunar cycles. Tishri 15th would fall on different days on our Gregorian calendar. The Roman calendar in Christ's day was the Julian calendar, which was changed to the Gregorian calenda