The Exact Day is Unknown
The exact date of Jesus' birth is not given. This is very likely because, if we knew it, we would reverence the day, rather than the Man. However, we can come to within about thirty days or so of it, and we know for certain that Jesus was not born on Christmas.
Narrowing the Range
First, a list of some of the guiding concepts that can help us to pinpoint, and narrow, the actual range of time (some of this for the month and some for the year):
The timing of Mary's pregnancy compared with that of her cousin Elisabeth
The timing of Zachariah's priestly duties, as these followed a regular and long-established schedule
The prophecies of Daniel 9 which foretold the 7-year period during which the Messiah would minister, being "cut off" in the midst of that time (after just 3.5 years of it)
The timing of Passover when Christ was crucified
Because a full exposition of each of these points would be lengthy, I will give some of the dates and times, and scriptures which must be studied to understand those times, so that the reader who is unsatisfied with the calculations can undertake to calculate them personally--and learn more in the process.
Mary conceived, and went to visit Elisabeth, her cousin, whom she had heard was also pregnant in her old age.
24 And after those days his wife Elisabeth conceived, and
hid herself five months, saying, 25 Thus hath the Lord
dealt with me in the days wherein he looked on me, to take away my
reproach among men. 26 And in the sixth month the angel
Gabriel was sent from God unto a city of Galilee, named Nazareth,
27 To a virgin espoused to a man whose name was Joseph, of
the house of David; and the virgin's name was Mary. (Luke 1:24-27,
36 And, behold, thy cousin Elisabeth, she hath also
conceived a son in her old age: and this is the sixth month with her,
who was called barren. 37 For with God nothing shall be
impossible. 38 And Mary said, Behold the handmaid of the
Lord; be it unto me according to thy word. And the angel departed from
her. 39 And Mary arose in those days, and went into the
hill country with haste, into a city of Juda; 40 And
entered into the house of Zacharias, and saluted Elisabeth. (Luke
Mary's pregnancy is plainly spelled out to be six months' behind that of Elisabeth, placing John the Baptist six months older than Jesus, being born six months ahead of Jesus.
Zacharias' Course of Service in the Temple
Every priest coveted the opportunity to minister in the temple. To bring order and fairness to the system, David was the first divide the priests and Levites up by courses to serve on a regular schedule. Later kings confirmed and continued this program of service.
3 Now the Levites were numbered from the age of thirty
years and upward: and their number by their polls, man by man, was
thirty and eight thousand. 4 Of which, twenty and four
thousand were to set forward the work of the house of the LORD; and
six thousand were officers and judges: 5 Moreover four
thousand were porters; and four thousand praised the LORD with the
instruments which I made, said David, to praise therewith.
6 And David divided them into courses among the sons of
Levi, namely, Gershon, Kohath, and Merari. (1 Chronicles 23:3-6, KJV)
The instructions which follow in the next chapter (1 Chr. 24) show the courses to have been laid out like this:
||Course / Family in Charge
||Course / Family in Charge
|1 - Abib/Nisan
|2 - Zif
|3 - Sivan
|4 - (Tammuz)
|5 - (Av)
|6 - Elul
|7 - Ethanim
|8 - Bul
|9 - Chisleu
|10 - Tebeth
|11 - Sebat
|12 - Adar
(NOTE: Parenthesized names in the table are not explicitly given in the Hebrew.)
Zacharias is said to be of that eighth (8th) course: "Abijah" (Hebrew) / "Abia" (Greek).
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. (Luke 1:5, KJV)
With 24 courses, this means there can be two courses per month. This would mean Zacharias would serve in the fourth month. The first month was defined as the start of the Jewish year. It was the month of Passover.
In the fourteenth day of the first month at even is the LORD's
passover. (Leviticus 23:5, KJV)
The Passover usually came around the end of March or the beginning of April by our modern calendars--it was spring time. Assuming roughly mid-April for the Passover, and the first of the month being two weeks earlier, the eighth course, that of "Abia," should be expected to fall around the latter half of July (from mid-month to its end). It would have been after this that Zacharias returned home, and if conception happened shortly thereafter, John could be expected roughly nine months later, around April of the following year. This would time Jesus' life and ministry perfectly, as he is to be born six months later, and it was at Passover time, 3.5 years into his ministry, that he is crucified (aged 33.5, because his ministry is said to begin when he is thirty and is baptized by John).
Which brings us to the 70-weeks' prophecy of Daniel 9.
The Seventy Weeks / 490 Years
The prophecy is given in Daniel 9 as follows.
24 Seventy weeks are determined upon thy people and upon
thy holy city, to finish the transgression, and to make an end of
sins, and to make reconciliation for iniquity, and to bring in
everlasting righteousness, and to seal up the vision and prophecy, and
to anoint the most Holy. 25 Know therefore and understand,
that from the going forth of the commandment to restore and to build
Jerusalem unto the Messiah the Prince shall be seven weeks, and
threescore and two weeks: the street shall be built again, and the
wall, even in troublous times. 26 And after threescore and
two weeks shall Messiah be cut off, but not for himself: and the
people of the prince that shall come shall destroy the city and the
sanctuary; and the end thereof shall be with a flood, and unto the end
of the war desolations are determined. 27 And he shall
confirm the covenant with many for one week: and in the midst of the
week he shall cause the sacrifice and the oblation to cease, and for
the overspreading of abominations he shall make it desolate, even
until the consummation, and that determined shall be poured upon the
desolate. (Daniel 9:24-27, KJV)
Using the prophetic principle of a day representing a year (a natural understanding for Hebrew speakers since the word "yowm" in Hebrew can mean day, time, or year) as found in Ezekiel 4:6 and Numbers 14:34, the 490-day period is found to span 490 years, and this begins with the decree to restore and rebuild Jerusalem. This decree is established in history as having been proclaimed in 457 B.C. The first "seven weeks" of this prophecy, representing 49 years, indicate the time during which Solomon's temple was to be rebuilt. Owing to political problems, work had stopped for about three years, but the Jews give us the correct figure of how many years the building had actually occupied.
Then said the Jews, Forty and six years was this temple in building,
and wilt thou rear it up in three days? (John 2:20, KJV)
So the temple was built during that first seven-week (49-year) period of the prophecy. The last week (seven years) of the prophecy is split in the middle by Jesus' death; the first part being the period of his earthly ministry, and the last half being the final years of probation for the Jewish nation before God turns from them to the Gentiles at the stoning of Stephen.
To arrive at the year of Jesus' birth requires simple subtraction (working backwards) from the end of the prophetic period and/or from the time of the crucifixion.
The 490-Year Period in Reverse-Chronological Summary
A.D. 34 -- stoning of Stephen / end of the 490 years
A.D. 30/31 -- Jesus is crucified, causing the sacrificial system to cease
A.D. 27 -- Jesus is baptized at age 30 (see Luke 3:23)
4 B.C. -- Jesus is born (remember, there is no zero year between the B.C. and A.D. dates)
457 B.C. -- the start of the 490 years, and the start of the rebuilding of the temple in Jerusalem
The Timing of Passover in Year of Crucifixion
The Passover lamb, whose type Jesus must fulfill, was always eaten on the day of Passover, at the start of that day, which biblically is evening, following sunset. The Passover was considered a sabbath, even when it did not land on the seventh day of the week; and sabbaths were always observed from sundown to sundown. In A.D. 30, Passover came on a Friday. Thursday evening, at the start of Passover, the passover lamb was eaten in the upper room by the disciples. Thursday night found them in the Garden of Gethsemane, where Jesus began to bear the sins of the world, placed on him as the Lamb of prophecy. He was arrested, taken to Pilate, then to Herod, then back to Pilate before being sentenced to crucifixion well into the morning on Friday (Passover), and Jesus himself was sacrificed and died on this same day. He rested in the tomb on the Sabbath, the day following, and was resurrected on the first day of the week (Sunday morning).
Some Finer Details
Shepherds did not stay outdoors through the night with their sheep during the winter months. That was the rainy season, and would have been the time when crops would be growing in the fields. The earliest rains might begin in September or October, and by November, the fields would have been under agriculture again, having lain fallow during the dry months of summer from about May/June through September/October. See HERE for some details.
This eliminates the winter months, beginning around November and continuing through at least April/May, as a time when shepherds would have been out keeping watch over their flocks by night; and the shepherds were on duty when Jesus was born (see Luke 2:8).
Why 30/31 AD?
The calendar reckoning we use today does not match the reckoning used in Jesus' time among the Jews. As God had established, they observed the first month of the year (Nisan/Abib) in about April, celebrating Passover in the midst of that month. Because we, today, start the year on January 1, this leaves some ambiguity as to which "year" is being addressed. Many such dates are denoted, therefore, as a range, as in this case "30/31." It would be A.D. 30 as of January 1, but the Jews would have considered it A.D. 31 when Jesus was crucified, on Passover, two weeks into their New Year.
In fact, from about April onward for each and every year discussed here, the problem is the same--which is one source of confusion to many people.
By far the strongest evidence in favor of a September/October birth is found in a study of the priestly courses and following the timing of Zacharias' course, the birth of John the Baptist, and the birth of Jesus six months later. Zacharias would have served in July, and assuming Elizabeth was pregnant in August, John would have been born in April, leading to Jesus' birth in about September/October, six months later. Naturally, these calculations assume that the period of gestation was essentially the same in the days of Christ as it would be now (about 36-40 weeks; nominally 9 months).
Given a late September/early October date for Jesus' birth, all other prophecies relative to the time of the Messiah and Christ's ministry are well aligned. It is virtually certain, therefore, that Jesus was born in the early to mid-autumn of 4 B.C.