Why was December 25th chosen as the day to celebrate Jesus' birth?
This is a good recent blog post covering Pope Benedict's thoughts on the matter.
The key is the date set for the annunciation (March 25th):
Therefore, as vsz says wikipedia says, but should say louder, add 9 months, et voila you've got December 25th.
As for the age of the feast as celebrated by Christ's Church, said blog post also notes St. Pope Leo the Great mentioning the feast of the Nativity of Jesus in the winter:
I'm not going to say that only stupid people think that Christmas is an attempt to quash pagan solstice shenanigans, but I will say that people ought to think long and hard before believing the Discovery channel over thousands of years of tradition.
The exact day for the birth of Jesus is not known. You can find a lot of info on Wikipedia, with references. The most interesting for this question might be:
"Chronograph" 1 – a document dating to 354 AD 2 in Rome: This was a list of martyrs and their birth dates, and a list of bishops of Rome and their birth dates. The Chronograph lists these dates (birth dates) in calendar order. The first date listed is the "8th of Kalens of January" ("Kalens" is used to refer to the first of a month – putting a number in front of it counts backward.) This translates to our December 25th. The notation in the Chronograph is that this is the birth of Christ in Bethlehem.
On the list, the last two bishops were added on apparently after the list was otherwise complete (dates out of order). The first bishop had died in 336 AD. Thus, it is probable that this list dates to 336 AD.
In 303 AD, under Diocletian, was the "last" persecution of the church. After this time of persecution, lapsed Christians-those who had recanted under persecution, were allowed back into the church (upon profession). There was a group called the Donatists that disagreed with this practice – they felt that the lapsed Christians should not be allowed to rejoin the church. The Donatists broke away from the church.
Augustine opposed the Donatists. He noted 3 that "the Donatists do not celebrate Epiphany", which was started as a celebration of the baptism of Jesus. Apparently, the Donatists celebrated Christmas – Augustine doesn’t say that they didn’t. This would push the celebration of Christmas back to before 311 – the time of the Donatist split.
There were several assumptions made in calculating this date – assumptions from Jewish tradition. Jewish tradition held that the time of the Passover celebration was also the time of the beginning of creation. The sun, moon, and stars were created on the 4th day of creation. Malachi 4:2 refers to the "Sun of Righteousness" (referring to Christ). The reasoning is that it was symbolically fitting for March 28th to be the birth of the light of the world.
Though use of December 25th is not supported, yet the plan of celebrating birthday of Jesus was prevailing around 243 AD.
Note that even as early as the 12th century, it was thought that December 25th was co-opted from the pagans. EVEN IF it was co-opted, it was infused with new meaning by the best theologians of the time. But there is good evidence that a date of December 25th is true – it just happened to fall on the winter solstice. Early Christians believed that December 25 was the true day. It is likely that the early Christians would try to identify the birth day of their Saviour.
"De Solstitiis" – a North African document from the 300’s attempts to determine the date of Christ’s birth entirely from the evidence given in Scripture. It assumes a date of March 25th for Christ’s death (Passover).
The reasoning begins from the conception of John the Baptist. The angel made the announcement to Zechariah. Zechariah completes his "time of service" and returns home. (Luke 1:23-24) The time of service is complete in the fall of the year (when the Jewish year changes). When Zechariah returns home, Elizabeth conceives John. This is around the time of the Fall Equinox.
The announcement to Mary comes in Elizabeth’s six month – this would be about the Spring Equinox. This also happens to be Passover – the date of the death of Christ – March 25th (by the Julian calendar).
Birth of John – 9 months after Fall – Summer solstice.
This reasoning is based on the presumption of the conception of John the Baptist.
This line of reasoning is adopted by Augustine – he notes that the conception and crucifixion occurred on the same day. Also accepted by John Chrysostum – in the 380’s, he reproduces this reasoning in the east – using the common eastern calendar – winds up 2 weeks off – January 6 for the birth of Jesus and April 6 for the death of Jesus. Augustine and Chrysostum promoted the celebration of December 25 because it is believed to be the actual date.
Since Clement of Alexandria was in the "east," we can assume that he used the eastern, or Egyptian, calendar in his calculations. He arrived at a date of January 6th (according to our calendar). This is the reason for the eastern church using Jan 6 and resisting December 25.
Also, Epiphanius (died 403 AD) in Palestine does the same calculation as in De Solstitiis, using the eastern calendar for dates, and also winds up two weeks later than December 25.
All these calculations have nothing to do with pagan festivals or equinoxes and solstices. (Jan 6th is two weeks after the winter solstice.)
Apart from above I also stumbled upon a strong biblical support for this date:
Jewish tradition use two kinds of calenders, one is Civil calender which is meant exclusively for kings, childbirth n contracts. The second was Sacred calender for religious festivals. Following are the months of Jewish Civil calender:
Tishri overlaps on Sept. to Oct. and is the first month.
Heshvan overlaps on Oct to Nov,
Chislev overlaps on Nov to Dec. and so on;
Next is Tebeth,
Now Luke in verse 1:26 says that it was in the sixth month angel Ganriel was sent to Mary, meaning it was month of Adar which overlaps on Feb to March. If we take this as the time of Jesus' conception then we come close to Nov to Dec after nine months. So this is a strong biblical support for Christmas to be in December.
Further Luke says at verse 1:36, that it is now sixth month for Elisabeth's pregrency, meaning St. John the Baptist was born in the month of Sivan which is somewhere June and that is how we are celebrating his day on 24 th June.
Let me add here: We do not find any historical date in Bible, may be for obvious reason since the bottom line in Bible was to deliver a message of Salvation and not a historical record of events that took place. Notwithstanding this, there is always an urge in humans to remember and celebrate those events, which influenced their destiny. That urge gradually led the early Christian to commemorate all those sacred events that took place in life of our Lord as remembrance and veneration of our Lord. Savior and Emmanuel (God with us) spent His life as one of us and like us among us the earthly creatures. His birth, death and whatever mighty deeds He carried out are by no means any ordinary events.
Suppose we were those unfortunate who never knew the birth date of one of our parents, we would somehow try to decipher the probable date from whatever source. As such not knowing the date would not stop us from celebrating this event. In similar scenario, we were unfortunate not to know the exact date of our Lord's birth and in absence of any exact dates in Bible, it is not at all a SIN to fix appropriate day for this event in a year to give us opportunity to commemorate them in honour of our Lord. If one ponders it in an unbiased mind, it is not an issue at all whether Christian celebrate the Christmas on 25 December or 6 th of January as some do. What is important is we need one day to celebrate the day of birth of our Savior. Whether it is the date which coincides with the birth day of some pagan god or with birth of any other founder of any other present day religion is immaterial for any true and broad minded Christian. Celebrating a birthday by no means can be considered as Pagan.
Because paganism was prevailing during the early years of Christianity, just by relating any event in Christian practice to a pagan practice is easiest way to create doubt in minds of believers by saying that it relates to pagan practice.
1 The Chronography of 354. Introduction to the online edition
2 The Manuscripts of the "Chronography/Calendar of 354 A.D."