This is one of those questions to which the simple answer is – We don’t know?
Tradition has it that it was on the 25th of December. Some say that this was to Christianise a pagan festival, which is perhaps a cynical view, and others say that this was the adoption of a date which was already widely accepted and celebrated.
We might say that this is a fitting celebration of the birth of the “Light of the World” during the dark days of winter bringing some cheer to an otherwise cold and gloomy time of the year.
On looking in the Catholic Answers website a number of other dates are also mentioned;
-200AD Clement of Alexandria noted that Christians in Egypt believed Jesus was born on May 20.
-De Paschae computus, 243AD, notes that some Christians celebrated Jesus’ birth on March 28th and others on April 19/20th.
-Theophilus and Hippolytus (2nd and 3rd centuries respectively) mention December 25th.
-4th and 5th Century writings mention December 25th as an ancient and widespread feast of Christ’s birth.
-Many ancient writings, especially in the east, place the celebration on January 6th.
-The Church chose December 25th due to its ancient origin and widespread acceptance but is not declaring de fide that it is the literal date he was born.
Do we have any biblical clues?
Luke 2:8-11 – In the countryside close by there were shepherds who lived in the fields and took it in turns to watch their flocks during the night. The angel of the Lord appeared to them……..”Today in the town a David a saviour has been born to you; he is Christ the Lord,”
If this was on the 25th of December, winter, would the sheep have been out in the fields?
In Luke Chapter 1 we have some other clues. Zechariah is serving in the temple according to a rota as a priest of the Abijah section of the priesthood. This section is the eighth out of a total of 24 listed in 1 Chronicles chapter 24. The angel of the Lord appears to him and says that he and his wife Elizabeth will have a son whom they must name John (John the Baptist – cousin of Jesus). When his turn on the rota was over he went home to his wife and she conceived. If we knew when his turn on the rota finished we would be able to fix the approximate time of year, month, when Mary visited Elizabeth after the annunciation and her yes to God, because the angel told Mary that her kinswoman Elizabeth was six months pregnant. Luke 1:39 tells us that Mary set out to see Elizabeth with little delay.
When Mary and Elizabeth meet Elizabeth confirms that Mary is pregnant – Luke 1:39-45.
So if we knew when this was by the guidance of Zechariah’s temple service rota we would be able to project forward six months and then nine months for an approximate date for Jesus’ birth.
claims this has been worked out putting Zechariah’s temple service during the Hebrew month of Sivan (May June overlap) meaning John was conceived towards the end of Sivan or in the following month of Tammuz (June – July overlap). Six months from here takes us to Kislev (Nov-Dec overlap) and a date for the Immaculate Conception. Nine months from here brings us to Tishrei (Sept – Oct overlap) and Jesus being born in the Autumn round about the Feast of Booths or Tabernacles (Succoth), which is being celebrated now – October 2nd to 9th 2020.
What is the significance of this?
John 1:14 – The Word was made flesh and dwelt (was tabernacled) among us.
I have come across this in other Christian sources but so far have not found it in a Catholic source. So I only raise it as something of interest.
For myself it gives me an opportunity to reflect on Jesus’ birth “being tabernacled among us” away from all the commercial hype of Christmas and I will celebrate Christmas in December in spite of all these trappings.
Isaiah 7:14 – Therefore the Lord Himself will give you a sign: Behold, the virgin will be with child and will give birth to a son, and she will call Him Immanuel.
Matthew 1:23 – “The virgin will conceive and give birth to a son, and they will call him Immanuel” (which means “God with us”).