At this time of year, the TV and radio constantly remind us of the number of days, or, as excited children term them, 'sleeps', to Christmas. This can be calculated because 25 December is now generally agreed upon as the date of Christmas Day, although some Eastern Orthodox churches, such as the Russian Orthodox Church, celebrate the day on 7 January.

The word Christmas means 'Christ's Mass', and was first recorded in 1038 as the Old English 'Cristes maesse', reminding us that the day is a celebration of the birth of Jesus Christ. But over the centuries, there has been much uncertainty about when exactly that birth happened, since the New Testament accounts of the birth of Jesus give no date.

One of the earliest mentions of the feastday of the birth of Jesus comes from early Christian theologian Clement of Alexandria, who wrote of it being celebrated on 20 May. The first known reference to 25 December as the date of the birth of Christ is found in a manuscript put together in Rome some 354 years after that birth.

Clement of Alexandria

In Judeo-Christian tradition, God created the world at the time of the spring equinox, which was 25 March on the ancient Roman calendar. This date is now celebrated by Christians as the anniversary of the Annunciation, when Mary was told by an angel that she would give birth to the son of God. Counting nine months ahead from that date would have left the early Christians with a date of 25 December for that birth.

As Christianity developed in Europe, the importance of Christmas led to it replacing the Epiphany on 6 January as a day of huge significance. This was particularly clear when Christmas Day was linked to political power in Christian Europe. In the year 800, it was on 25 December that Charlemagne was crowned Holy Roman Emperor, establishing a sense that God supported the transfer of Christianity's power base from Byzantium; in 1066, after the Norman conquest of England, the same date was chosen for the coronation of the Norman leader, Duke William, as King William I.

A bust of Charlemagne

Eighteenth-century English philosopher and scientist Isaac Newton argued that the early Christians had chosen the date of 25 December to fit in with the time of the winter solstice - a pagan festival celebrated at the same time of the year by the ancient Romans, and pagan tribes in Europe. This festival marked the point of midwinter, when the earth was farthest from the sun.

Isaac Newton, painted in 1689 by Godfrey Kneller

In 1743, the philosopher Paul Ernst Jablonski connected the date with the ancient Romans' celebration of the rebirth of the sun, as from this point the earth began a new cycle that brought it closer to the sun's warmth again and the return of life to the frozen land.

The number of 'sleeps' to Christmas preoccupies us at this time of year. In reality, the birth of Jesus could have happened at any time. But as we prepare to celebrate in winter, we are following traditions already well-established at the time of that birth.