The oldest written testemony for a Christmas feast on the twenty-fifth of December is from the Church of Rome and dates back to about 330. In the calendar ascribed to Furius Dionysius Filocalus we have two lists (depositiones) from the year 336, one with the anniversaries of the death of the bishops of Rome (depositio episcoporum), the other one with the anniversaries of the martyrs (depositio martyrum). In the list of the martyrs we find the remarks: "VIII kal. jan. natus est Christus in Bethlehem Judaeae" which means literally translated "Eight days before the calendae(first day) of January Christ is born in Bethlehem of Judaea". It is therefore certain that the birthday of Christ was celebrated in the Church of Rome on the Twenty-fifth of December in 336.
Why the twenty-fifth of December? The Gospels did not give us any indication about the day on which Christ was born. There are mainly two theories; one is so-called "calculation hypothesis", and the other one the hypothesis from the history of religion. The calculation hypothesis got its name because it is based on the calculation of the Church Fathers who as early as the third century began to speculate about the date of the birth of Christ. The problem with this theory is that the calculations of the Church Fathers differ very much from each other and sometimes are difficult to interpret. They are very much connected with the Christ-sun symbolism and therefore related to the equinoxes and solstices. In a simplified way the most widespread version of the "calculation hypothesis" says that Christmas is celebrated on the twenty-fifth of December beacause redemption began on the twenty-fifth of March.
The widely accepted theory is from the history of religion which considered the pagan feast " Natale solis invicti " (Birthday of the invincible sun) as the basis of the Christian Christmas feast. That pagan feast was introduced by Roman emperor Aurelian in 274 to be observed on December 25, the day of the winter solstice. The Christians accepted this pagan feast but gave it a Christian content: it became the birthday of their true and invincible Sun Jesus Christ. It seems that such aChristianization process was not easy because even hundred years later Pope Leo the Great (440-461) in his Christmas sermons observed that there are still Christians who do not observe this day as holy because of Christ but because of the sun. From Rome the feast of Nativity of the Lord spread rapidly to other places and almost in the whole world.