However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar (although initially, namely between 25 BC and AD 5, it was unsynchronised with the original implementation of the Julian calendar which was erroneously intercalating leap days every third year due to a misinterpretation of the leap year rule so as to apply inclusive counting).
The 25 December Nativity of Christ was alleged very early by Hippolytus of Rome (170–236) in his Commentary on Daniel 4:23: "The first coming of our Lord, that in the flesh, in which he was born at Bethlehem, took place eight days before the calends of January, a Wednesday, in the forty-second year of the reign of Augustus, 5500 years from Adam."
[note 1] "Another early source is Theophilus Bishop of Caesarea (115–181): "We ought to celebrate the birth-day of our Lord on what day soever the 25th of December shall happen.
John Chrysostom, in a sermon preached in Antioch in 387, relates how the correct date of the Nativity was brought to the East ten years earlier.
Considering that Jesus was thought to have been conceived on New Year's Day of the Old Style calendar, 25 March was recognised as the Feast of the Annunciation which had to be followed, nine months later, by the celebration of the birth of Christ, Christmas, on 25 December.
The choice would help substitute a major Christian holiday for the popular Pagan celebrations surrounding the Winter Solstice (Roman Sol Sticia, the three-day stasis when the sun would rise consecutively in its southernmost point before heading north, 21, 22 and 23 December.
In AD 274, Emperor Aurelian had declared a civil holiday on 25 December (the "Festival of the Birth of the Unconquered Sun") to celebrate the deity Sol Invictus.
Finally, joyous festivals are needed at that time of year to fight the natural gloom of the season (in the Northern Hemisphere).
"[14] At the Council of Nicaea, it became one of the duties of the patriarch of Alexandria to determine the dates of the Easter and to announce it to the other Christian churches.
The rules to determine this are complex, but Easter is the first Sunday after a full moon occurring after the northern vernal equinox, which falls on or after 21 March in Alexandria.
When Julius Caesar reformed the calendar, the northern vernal equinox was nominally on 25 March which was abandoned shortly after Nicaea.