It is true that we do not know the exact date when the baby Jesus was born. There is no written record like a "birth certificate," but from the Scriptures and other historical data, we can come close to the year and an interesting reason for the date of December 25. At the time, the country of Israel was under Roman domination and thus subject to the requirement of Caesar Augustus that all the Roman Empire should be registered for taxation. The year of this registration was in the first term of Quirinius (Cyrenius), governor of Syria which was 4 BC to AD 1 (Luke 2:1-2). King Herod the Great, who ordered the murder of the infants in the area of Bethlehem (Matthew 2:13-23) died in 4:BC. (From The New Unger's Bible Dictionary, Revised, 1988, Moody Press). But these dates are uncertain, due to the difficulty of reconciling dates of events according to the Roman calendar with 365 days to the year and the Hebrew reckoning with 360 days. Also many records are incomplete or lost. The year of Christ's birth is now generally thought to be about 5 to 4 BC on our calendar.
The setting of December 25 as Christ's birthday is interesting, however. This was done by agreement of the churches in the western part of the Roman Empire many years later. They felt that the fact of Christ's birth, the incomparable occasion when God came to dwell within a human body, should be commemorated, as was the birth of many lesser members of the race. Though this date was an ancient heathen holiday, it was also the 25th of the month Chislev on the Hebrew calendar, corresponding to our December.
It was on this date that the Syrian king, Antiochus Epiphanes, defiled the temple in Jerusalem and dedicated it to his god, Zeus, and three years later on the same date, the 25th of Chislev, after the Maccabean victory, the temple was cleansed and rededicated to Jehovah. This began the Jewish holiday of Hanukkah (Dedication), later also called "The Feast of Lights."
The nine branched candlestick associated with Hanukkah is based on the tradition of the miraculous provision of enough consecrated oil to light the menorah for the eight days of the celebration. After Christ's death and resurrection, the Hebrew Christians in the years before the destruction of the temple in 70 AD made much of the many lights used in the city during this holiday week. They would naturally remember the words of Jesus as He said, " I am the light of the world; he that follows Me shall not walk in the darkness, but shall have the light of life" (John 8:12). Jesus is mentioned as the light of the world in John Chapters 8, 9, 11 and 12, and in the 10th chapter He is in the temple during the feast of the Dedication (10:22). Thus the 25th of the month Chislev was in mind when a birthday date for Christ was established.
There are some who claim that the nativity could not have been in December because shepherds did not keep their flocks outside at night in winter. But the flock mentioned in Luke 1:8 was special. These sheep were destined for temple sacrifices and were kept in a pasture close to Bethlehem. Their shepherds were not ordinary shepherds, and watched their flock outside all the year around (from "The Life and Times of Jesus the Messiah" by Alfred Edersheim). Hanukkah is observed today according to the Hebrew calendar and thus the date of the month varies compared to the Gregorian calendar currently in use.
More important than the date of Christ's birth are the recorded facts of it. These are clearly established as genuine history in Matthew and Luke and are in exact fulfillment of many Old Testament prophecies. The basic truths of Christ's birth, His death and His resurrection from the dead are vital for saving faith, and His own prophecy that He will return to earth assures eternal life for every believer and eternal judgment for those who do not believe. One other date of supreme importance to every member of the human race, therefore, is found in 2 Corinthians 6:2. "For He (God) says, 'at the acceptable time I listened to you, and on the day of salvation I helped you;' behold, now is 'the acceptable time,' behold, now is 'the day of salvation'. And Romans 10:13 assures us that "Whoever will call upon the name of the Lord will be saved."