The Gospel of Matthew indicates that Jesus was born before the death of Herod the Great (Matthew 2:1, 15, 19). New Testament scholar Donald A. Hagner, in Matthew 1-13 of the Word Biblical Commentary, notes: “Herod, whose long reign began in 37 B.C., died in 4 B.C.” Most modern scholars agree.
What is the evidence? The first-century Jewish historian Flavius Josephus, in his Antiquities of the Jews (17.6.4), tells of a lunar eclipse in the final year of Herod’s life. Solar and Lunar Eclipses of the Ancient Near East by Kudiek and Mickler states that this eclipse occurred March 13, 4 B.C. Josephus also reports that Herod’s death occurred before the next Passover (Antiquities 17.9.3). This Passover—in April of 3 b.c.—was 13 months after the lunar eclipse and, according to Jewish tradition, a few months after Herod’s death. Taken together, this information confirms a birth date for Jesus in the latter half of 4 b.c.
How could Jesus have been born in a year b.c. —“Before Christ”? The 4 b.c. -a.d. system of reckoning time was devised in the sixth century a.d., hundreds of years after Jesus’ birth. At that time, a misunderstanding of the chronology of Herod’s reign caused a miscalculation of Christ’s birth year, an error that was detected too late to be corrected.
As for the month and day of Jesus’ birth, the Gospels make no specific statement; Jesus’ birth was not commemorated in the early days of the church. Christmas was not included on the church calendar until the fourth century.
Some people have claimed that Jesus was born near the fall festivals. That is possible, but it is not proven. Luke 2:1-3 says that “everyone went to his own town to register.” Why would “everyone” go to such trouble? Apparently it was required. However, it is not likely that Rome would risk a rebellion by requiring each person to go to his own city at the same time as the local religion required everyone to go to Jerusalem. Most likely, an empire-wide census would take several years, and would be administered locally, by local customs, taking into account local religious festivals.
Some people have objected to the idea that Jesus was born in December, since there were shepherds staying in the fields (Luke 2:8), and shepherds supposedly didn’t do that in winter. (I have never seen this documented.) The weather in Bethlehem is sometimes cold, but sometimes mild in December, and there would have been a practical need to keep sheep somewhere near Jerusalem for sacrificial purposes. This doesn’t prove that Jesus was born in December, but it shows that this objection to a December birth isn’t conclusive.
In the early third century (long before Constantine), Julius Africanus and Hippolytus came up with December 25 as the date of Jesus’ birth. They don’t tell us how they came up with this date, but John Chrysostom does. His calculation may have been innocent, or it may have been contrived. We do not know what his motive was. Therefore, we cannot say that the December 25 date was contrived simply because a pagan festival already existed on that date.
When the church first began celebrating Christmas, it had nothing to do with trees and holly and reindeer. All those were added centuries later in northern Europe. The fact that non-Christian customs were later associated with the festival does not prove that the date itself originated in paganism. It may have been based on calculation instead.
However, for the moment let us suppose that Christmas originated as a deliberate substitution for Saturnalia, a pagan holiday. Many of the people who attended church were recently-converted pagans. Some were not-yet converted pagans. They were attracted to the Saturnalia festivities, and sitting at home alone was not a desirable option when merrymaking could be heard in the streets all around. So, the theory goes, the church provided a clean alternative: going to church.
Would it be wrong to have a church service in deliberate opposition to Saturnalia? Of course not. There is no question of the church trying to worship God by the customs of the heathen — the church is fighting against the customs of the heathen. Only the date is the same, and there is good reason to have church services on that date, on which members can invite their unconverted friends and family into church and away from paganism. At some point, Christians could have made the comparison: on this date, pagans celebrate the birth of the sun god, but we are worshipping the sun of righteousness (Malachi 4:2). We can celebrate his birth, too.
That may have been the way Christmas started. Apparently in the early centuries it was primarily a church service. The strategy seems to have been successful: no one celebrates Saturnalia any more. Christians don’t observe Christmas in honor of the sun god, just as they don’t worship the little statues that they may have in their homes or gardens. Although December 25, like many other dates, was once used for idol worship, that is not its meaning now.