Why is Christmas celebrated on December 25?

25th December has for Christians a deep religious value, but in the last decades it has become a more secular celebration, almost a "business", also in areas as India, China, Japan, Malaysia, where Christians are just small minorities.
Traditional Christmastime
The majority of the Christian population in the richer countries feel this is a time for family reunions, for giving and receiving presents, eating together, writing letters to Santa Claus, and the Christmas spirit through movies, carols, traditions evokes a wish for peace, love, forgiveness, memories and nostalgia.

The date of December 25th

There is no document, no papal bull or council that established the historical date of the birth of Jesus. Eastern and Slavic Orthodox churches celebrate Christmas on 6 and 7 January, coinciding with the Epiphany, still following the Julian rather than the Gregorian calendar.
Traditional Christmastime

Christmas is certainly the most beloved holiday in the year, but it is not the main religious event of Christianity: if Jesus had not resurrected, he would have remained one of the many great sages in the history of mankind. Therefore, the most important Christian celebration is Easter, whose date is historically fixed and astronomically certain: the 14th day of the Jewish month of Nisan, just before the beginning of the Jewish holiday, that is at dawn on Sunday 9 April of the year 30, as well as the date of Christ's death: about 3 pm on Friday 7 April of the same year 30.
St. Luke's Gospel
St. Luke writing the Gospel

In the Gospels there is no reference to a date of birth for Jesus; moreover, only two out of the four evangelists speak of the birth and childhood of Christ, while the other two start their narrative from the beginning of his public life. Moreover, in the Christian tradition the "Dies Natalis" on which a saint is commemorated is not the day of birth, but of death, that is, the rebirth to eternal life. Since the date of the birth of Jesus is not mentioned in the Gospels, every attempt to infer the year or month and day from the accounts of Matthew and Luke cannot give clear, acceptable results, which means there are two choices left. Either

1. December 25 is a conventional date, chosen by Christians as the birth of Jesus Christ to counter the pagan festival of the Sol Invictus, and also to superimpose other celebrations as the Jewish Hanukkah and Celtic cults linked to the astronomical calendar, or

2. December 25 is actually the historical date of the birth of Jesus, a hypothesis based on the Gospel of Luke and on the archaeological studies of the Qumran manuscripts by Israeli professor Shemarjahu Talmon.

Dies Natalis Solis Invicti

On Wednesday, December 22, 1993 Karol Wojtyla, during the last audience before Christmas, said:
"Dear brothers and sisters, here we are again at Christmas, a liturgical solemnity that commemorates the birth of the Divine Savior, filling our souls with joy and peace. The date of December 25, as is known, is conventional. In pagan antiquity, the birth of the "Invictus Sun" was celebrated on that day, coinciding with the winter solstice. It seemed logical and natural for Christians to replace that festivity with the celebration of the one and true Sun, Jesus Christ, born on earth to bring men the light of Truth."
Pope John Paul II celebrating Christmas
Pope John Paul II celebrating Christmas as a young cardinal

Also Joseph Ratzinger, Pope Benedict XVI, in the general audience of December 23, 2009, said that
"the first to clearly state that Jesus was born on December 25th was Hippolytus of Rome, in his commentary on the Book of the Prophet Daniel, written about 204".
In his speech Ratzinger stated that the Christian feast had replaced the pagan "Sol invictus" celebrated on the solstice, the shortest day of the year after which days start to get longer and the Sun wins over darkness. For Christians, it was quite natural to transpose the symbolism of victory over darkness into the coming of Christ on Earth.
Pope Benedict XVI celebrates Christmas Mass in St. Peter's
Pope Benedict XVI celebrates Christmas Mass in St. Peter's

But in no way did the Church ever clarify this point, letting Christmas become established as a mere tradition. Actually, the solstice was celebrated in Rome within the Saturnalia, a week of festivities related to this astronomical event: gifts were exchanged, and also a sort of carnival was held, when roles were turned upside down and slaves could act as masters. Probably, the Saturnalia ended just before December 25th, and according to some sources the date of December 25th was their last day.

The celebration of the Sun's Nativity was decided by emperor Aurelianus, who had established a true solar cult, linked to the god Mithra and dating back to more than a thousand years before Christ; Aurelianus, who made the sun-god the main deity of his empire and used to wear a ray-shaped crown, consecrated the temple of Sol Invictus on December 25, 274, in a celebration that was called "Dies Natalis Solis Invicti" (=Birth Day of the Unconquered Sun) which was attached to and concluded the Saturnalia, the most ancient Roman festival.

Aurelianus coin
Roman coin from Palmyra, representing Aurelianus with a crown in the form of sun rays

In 330 AD the celebration of the Christian Christmas was introduced for the first time with an imperial decree by Constantine I, overlapping and/or replacing the pagan holiday of the Dies Natalis Solis Invictis. From Constantine onwards, the figure of Christ was in fact considered by the Empire the most suitable new religion for social control, and was identified with that of the Sun to be better accepted by the masses, especially the legionaries, who at the time were largely followers of Mithraism. In this way the birthday of Christ was moved to December 25 to make it coincide with the birthday of the sun god, and the "Birth Day of the Sun" became the "Birth Day of Christ".
In 337 Pope Julius I formalized the date of Christmas for the Christian Church (today divided into Catholic, Orthodox, Coptic, and Protestant in many denominations), as reported by John Chrysostom in 390, and as documented in the 'Roman Chronography', an illuminated calendar for the year 354 AD, produced by the calligrapher and illuminator Furius Dionysius Filocalus.
The Chronography of 354 AD (the calendar of Philocalus), Part 6: December
The Chronography of 354 AD (the calendar of Philocalus), Part 6: December

The original volume, still existing in the 8th-9th centuries, is lost, but copies were made at that time, the most complete and faithful are in a 17th-century manuscript from the Barberini collection (Vatican Library). On December 25 there is "N.INVICTI.CM.XXX" (=Birthday of the unconquered, games ordered, thirty races) which is the oldest literary reference to the pagan feast of Sol Invictus, as well as the oldest reference to Jesus' birth, "VIII kal. ian. natus Christus in Betleem Iudeae" (=Birth of Christ in Bethlehem, Judea").

But why is the sun "born" some days after the solstice? The "death" of the sun takes place at the winter solstice (22 or 23 December). In the following 3 days the sun does not rise by any degree - from which 'Sol stitium' (= sun stop), while on December 25 the sun rises by one degree, signaling the "resurrection" of the sun and from 25 December onwards the days become longer until the equinox in March.

Therefore, cosmic symbolism inspired the leaders of the Church of Rome to elect the end of the winter solstice, December 25, as the birthday of Christ, and the end of the summer solstice, June 24, as that of John the Baptist, integrated with their dates of conception nine months earlier on the spring and autumn equinoxes, respectively.

Pre-Christian festivals

Among the many celebrations in pre-Christian Europe, the Roman Saturnalia, the Celtic Yule and the Jewish Hanukkah have strong connections with Christmas.

First, as we have seen, from the 3rd century onwards the "Dies Natalis Solis Invictis" was celebrated in Rome on 25 December. It was the day of the Mithraic rites of the birth of the Sun and the end of the Saturnalia, a Roman festival celebrated in mid-December, which was the archetype of many Christmas customs, such as decorations, the exchange of gifts and lighting of candles.

 Romans celebrating Saturnalia, painting by Thomas Couture
"Romans during the Decadence", celebrating Saturnalia, painting by Thomas Couture, Musée d'Orsay

Secondly, among the Germanic and Celtic tribes the winter solstice was considered an important moment of the year. To celebrate the rebirth of the sun, these populations held their Yule festival, which, like other pagan celebrations, was later assimilated to Christmas. Yule was a period of rest and dances, which in Iceland continued to be celebrated throughout the Middle Ages, until Martin Luther's Reformation.

Thirdly, Hanukkah: in the Hebrew calendar the Jewish festival of the "Dedication" or "Festival of Lights" starts on 25 Kislev, and lasts for eight days. However, Hanukkah never falls on the same day and can go from the end of November to the end of December.

Traditions for Hanukkah: the menorah, pastries, dreidel
Traditions for Hanukkah: the menorah, pastries, dreidel

Jewish historian Titus Flavius Josephus narrates in his book, "Jewish Antiquities XII", how the victorious Judas Maccabeus ordered eight-day festivities after rededicating the Temple in Jerusalem that had been profaned by Antiochus IV Epiphanes. According to the Talmud, the wicks of the menorah - the six-branches ancient Hebrew lampstand made of pure gold and used in the portable sanctuary set up by Moses in the wilderness and later in the Temple in Jerusalem- miraculously burned for eight days, even though there was only enough sacred oil for one day's lighting. During Hannukah celebrations include lighting candles each night, singing special songs, reciting the Hallel prayer, eating foods fried in oil, playing the dreidel game, and giving Hanukkah gelt (chocolate coins) to children.

Talmon's Calculus

Thanks to studies on the Qumran manuscripts by Shemarjahu Talmon of the Hebrew University of Jerusalem, ("The Calendar Reckoning of the Judgment from the Dead Sea Scrolls", in Scripta Hierosolymitana, Volume IV, Jerusalem 1958, pp. 162-199) the date of December 25th might be considered 50% "historical".

In ancient Israel the members of the priestly class were divided into 24 groups each consisting of six priestly families. The 24 groups, alternating in an immutable order, were to perform liturgical service at the temple for a week, twice a year, with each of the six serving one day of the week and all serving together on the Sabbath. The family of Aaron was the eighth to provide liturgical service at the temple and one of the two weeks a year assigned to it was the last week of September.

The Gospel of Luke opens with the story of an old couple, Zechariah and Elizabeth (would-be parents of John the Baptist) resigned to sterility, one of the worst misfortunes in Israel. Zechariah belonged to the priestly caste of Aaron (Luke 1:67-79), and his wife Elizabeth was a relative [Luke 1:36] of the Virgin Mary. Luke reports that one day Zechariah, "officiating in the turn of his family", that is during the week when it was the duty of Zechariah's family line to serve at "the temple of the Lord", had a vision of the archangel Gabriel (Luke 1:8-11) who announced to him that, despite his advanced age, he and his wife would have a son, they were to call him John and this son was to be "great before the Lord".

Domenico Ghirlandaio, Annunciation to St. Zechariah in the Temple
Domenico Ghirlandaio, "Annunciation to St. Zechariah in the Temple", Cappella Tornabuoni, Florence.

The Gospel of St.Luke then states that six months later the same angel appeared to Mary: "Even Elizabeth your relative is going to have a child in her old age, and she who was said to be unable to conceive is in her sixth month: nothing is impossible to God "(Lk 1, 26-37).

If Jesus was born on December 25, Mary's conception was 9 months earlier. In fact, Christian calendars celebrate the annunciation of the archangel Gabriel to Mary on March 25. Though the catholic Church does not have a liturgical feast for the conception of John the Baptist, the ancient Eastern Churches celebrate it solemnly between September 23rd and 25th, six months before the Annunciation to Mary.

However, this logical succession of dates from the conception of John had been based on unverifiable traditions until some decades ago, when the Dead Sea Scrolls were discovered. Professor Shemarjahu Talmon, using the texts found in the Essenes library of Qumran, was able to establish in which immemorial, unchanging chronological order the 24 priestly classes followed one another, and related this order to our Gregorian calendar.

Two from the Dead Sea Scrolls
Two from the Dead Sea Scrolls at their location in the Qumran Caves before being removed by archaeologists (photo Abraham Meir Habermann).

Thus, the tradition of the early Judeo-Christian Church of Jerusalem which placed the announcement to Zechariah between 23 and 25 September is very likely based on history, and what seemed mythical takes on verisimilitude. A chain of events that extends over 15 months: in September the announcement to Zechariah, the day after the conception of John; in March, six months later, the announcement to Mary; in June, three months later, the birth of Giovanni; six months later, the birth of Jesus, December 25th.