In the roots of Christmas we have cultures and religions from Syria, Egypt, Mesopotamia, Persia, Arabia and ancient Rome itself.
On December 25 it is the date of birth and celebration of divine characters preceding the appearance of Christ :the God Horus Egyptian, the God Mithras Indo-Persian, the Babylonian God Tammuz / Yule and Shamas.
Also on December 25 the Invictus Sol Elagabalus was celebrated in Emesa and the Sun God Dusares / Helios in Petra.
The birth of Zarathustra and Khrisna, Dionysus, Adonis, Attis, the God Freyr, centuries or millennia before Christ is also credited to this date.
In the northern hemisphere, on December 22-23-24 the Sun seems to stop in the sky (the closer you are to the equator, the more evident the phenomenon is):it is the Solstitium (Sun still).
In astronomy these are those days when the sun stops to reverse its motion in the sense of DECLINATION ; it is the point where the Sun reaches the maximum distance from the equatorial plane. The darkness of the night reaches its maximum extension and the daylight the minimum. We have the longest night and the shortest day of the year. Immediately after the solstice, the daylight gradually increases again and the darkness of the night decreases until the summer solstice, in June when we will have the longest day of the year and the shortest night.
The ancient populations were well aware of this phenomenon of the solstice and transformed it into a festive occasion. This astronomical interpretation explains why the December 25 (and adjacent days) is a date present in very distant cultures and countries, from India to Mexico, from northern Europe to Ethiopia.
December 25th as a great day of celebration appears in Italy and Europe for the first time in 274 AD. by order of the Emperor Aurelian who made Christmas of the Sun an Official Holiday and wanted it to be celebrated throughout the Roman Empire:the Dies Natalis Solis Invicti.
The emperor Aureliano had just completed the reunification of the Roman Empire and had just returned from the great victory over the then main enemy of the empire, the Queen Zebedia of the Kingdom of Palmyra. The victory was made possible by the deployment of Emesa, a rival city-state, alongside the Roman army in a moment of disarray of the militias.
Aureliano proposed the Sol Invictus of Emesa to the Hellenic-Roman lovers of the Sun-Apollo, to the widespread followers of Mitra , to the Egyptians of the rites of Isis / Horus / Se rapide, to the Syrians and Arabs of the cults of Helios / Dusares and Baalim, to the Celts of Mastruca and to the Germanics lovers of Yule, the Wheel , unquestionable symbol of the Sun.
Particularly solemn were the celebrations of the rite of the birth of the Sun in Syria and Egypt:the celebrants retreated to special shrines from where they left at midnight, announcing that the Virgin had given birth to the Sun (depicted by Egyptians as a child).
The Sol Invictus festival established itself as the most important festival of the Empire, with great popular participation in Rome, also because it was grafted and ended the oldest Roman festival, the Saturnalia.
The Emperor Constantine , who was, and remained, also a lover of the Sun God, embracing the Christian faith transformed in 330 the feast of the Sol Invictus of December 25th into a Christian feast .
Previously (March 7, 321) Constantine had also changed the name of the first day of the week, a holiday:from Dies Solis (“the venerable“ day of the Sun) to Dominus, (Lord's day).
These changes were not always welcome, so much so that in central-northern Europe the ancient name of the day of the Sun has remained (Sunday among the Saxons , Sontag among the Germanics ).
In 337 Pope Julius I made the Catholic Church official the date of Christmas, as reported by St. Chrysostom in 390:
“On this day, December 25th, also the nativity of Christ it was recently established in Rome ”. In 354, the feast of Christian December 25 is mentioned for the first time in a calendar of the Roman liturgy. In 461 this choice will be confirmed by Pope Leo Magn or. Other ecclesiastical authors postpone the first apparition of Christmas in the West to 354 with Pope Liberius.
The choice of the Church of Rome to make the birth of Christ coincide with the most celebrated pagan festival it was an attempt to respond to the great participation that the cult of the sun kept among the population of the Empire , adapting it as a cult to the new religion.