While trying to find and answer to this question I came across this explanation by Andreas Köstenberger - October 4th, 2007. posted at http://www.biblicalfoundations.org/bible
It states that Jesus’ birth most likely took place in late November of 5 B.C. (the most authoritative treatment of which I am aware is Paul L. Maier, “The Date of the Nativity and the Chronology of Jesus’ Life,” in Chronos, karios, Christos: Nativity and Chronological Studies Presented to Jack Finegan [ed. J. Vardaman and E. M. Yamauchi; Winona Lake, IN: Eisenbrauns, 1989], 113–30). Based on a christian perspective if you look at Luke 3:23, Jesus was “about 30 years old” when he began his ministry. If Jesus was born in 5 B.C. (as argued above) and began his ministry, as is indicated by all four Gospels, shortly after that of John the Baptist (that is, in the latter part of the year A.D. 29), this would mean that Jesus was about 33 years old when he began his public ministry (see H. W. Hoehner, Chronological Aspects of the Life of Christ [Grand Rapids: Zondervan, 1977], 31–37 and B. Messner, “’In the Fifteenth Year’ Reconsidered: A Study of Luke 3:1,” Stone-Campbell Journal 1 : 201–11).
John’s Gospel records Jesus’ appearance at at least 3 Passovers: (1) in Jerusalem (2:13, 23); (2) in Galilee (6:4); and (3) again in Jerusalem (11:55; 12:1). In addition, it is likely that he attended a fourth Passover not recorded in John but recorded in the Synoptics (Matt 12:1 pars.?). This adds up to a length of about 3 ½ years for Jesus’ ministry. If he began his ministry in late A.D. 29, this brings us to A.D. 33 for the crucifixion. It so happens that because of astronomical calculations A.D. 30 and 33 are the only possible dates for Jesus’ crucifixion as far as the date of Passover in these two years is concerned (for the dating of the four Passovers in question see esp. C. J. Humphreys and W. G. Waddington, “The Jewish Calendar, a Lunar Eclipse, and the Date of Christ’s Crucifixion,” Tyndale Bulletin 43 : 331–51, esp. 335).
Finally, John 2:20 says that the temple was completed 46 years ago (see for this translation A. J. Köstenberger, John [BECNT; Grand Rapids: Baker, 2004], 109–10). According to Josephus, the renovation of the temple building proper started in 20/19 B.C. (Antiquities 15.11.1 §380), with completion 18 months later in 18/17 B.C. (Antiquities 15.11.6 §421). Again, counting from 18/17 B.C., adding 46 years brings us to A.D. 29 (there was no year zero)—a great way to check our math above!
Another source, http://wiki.answers.com, indicates that he was probably born about 4 B.C. Jesus was born on or before 4BCE, because Herod died 4BCE.
Jesus was born on or after 6CE, because Cyrenius became governor of Syria on 6CE:-
Luke2:2 And this taxing was first made when Cyrenius was governor of Syria. 2:3 And all went to be taxed, every one into his own city. 2:4 And Joseph also went up from Galilee, out of the city of Nazareth, into Judaea, unto the city of David, which is called Bethlehem; (because he was of the house and lineage of David:) 2:5 To be taxed with Mary his espoused wife, being great with child.
The definition of counting the year itself is being revised from time to time. It has a lot of changes throughout the history to make the year coincide with seasonal events of nature. For example we are not sure, as to whether there is a year of 0 AD , or 0 BC existed in our year counting sytem, the system itself came in existence much later than its reference point. But we still say about things happened years before and after Christ. There is modernisation of calender by a sudden jump of advancing the date in order to make the year more scientific or agree with astromical accuracy. With all these, the date of christmas we take as Jesus' day of birth. This may be in present system of Dec 25 or old system of Dec 25. When the present scientific way of recording was not in vogue we have take the available sources we trust. We are in the year , 2009 A.D and that itself is the best information.
It is sort of interesting that a person would ask or want to know that and if you follow astrology it will tell you that his birthday is in sept. but if you read the bible it gives you clue's as to his birth, it is said that when the magi's follow the star and it pointed to where jesus would be born there was no snow no rain nor was there sun for it was in the night that the angels came to the sheperd's to tell them Peace and good will toward men on earth for a child is born in bethlehem and if you read the sheperd's were watching and tending their sheep so your answer if you do a little research will be between late sept. or early oct. both months represent perfection in the sign of virgo and balance with peace in the sign of libre.