The first Church did not celebrate the birth of Christ. And the actual date of his birth was and still is unknown. The earliest known indication to such a celebration comes in a passing statement by St. Clement of Alexandria who mentions that the Egyptians of his time celebrated the Lord's birth on May 20. At the end of the 3rd century, the Western Churches celebrated it in the winter, and this was only accepted in Rome in the middle of the 4th century.
Around that time it was agreed by the Church all over the world to celebrate the nativity of our Lord Jesus Christ on 25 December, most probably to take the place of a pagan feast that even Christians continued to celebrate until then.
At that time, and until the sixteenth century, the civil calendar in use the world over was the Julian calendar, introduced by Julius Caesar in the year 46 B.C. This calendar considered the year to be 365.25 days 4 and thus had a leap year every four years, just like the Coptic calendar. Therefore, until the sixteenth century, 25 December, as the date of the celebration of the Lord's nativity.
Towards the end of the sixteenth century, Pope Gregory XIII of Rome took interest in studying astrology, dates and feasts. He noticed that the vernal equinox, the point at which the sun crosses the equator, making day and night of equal length, starting the spring, used to fall on 21 March (25 Baramhat) around the time of the council of Nicea (A.D. 325) which set the times for the ecclesiastical feasts. The vernal equinox at his time however fell on 11 March.
After consultation with scientists, he learned that the equinoctial year (or solar year), which is the time the earth takes to revolve around the sun from equinox to equinox, was slightly shorter than the Julian year. It was 365.2422 solar days (approximately 11 minutes and 14 seconds shorter). This makes a difference of a full day every 128.2 years, hence the difference of 10 days in the beginning of spring between the fourth and sixteenth centuries.
Pope Gregory XIII decreed the following:
· In A.D. 1582, October 5th will be called October 15th.
· The Julian calendar should be shortened by 3 days every 400 years, by making the centenary year a normal 365-day year, not a leap year, except if its number is divisible by 400.
· Thus the year 1600 remained a leap year as usual, while 1700, 1800 and 1900 had only 365 days each and the year 2000 was a leap year of 366 days.
· This new calendar came to be known as the Gregorian calendar, and is the common civil calendar in use in our world today.Following these decrees, as the Church of Rome celebrated Christmas 25 December 1582 A.D., the Eastern Churches still fasted as they showed 15 December or 19 Kiyahk on their Julian and Coptic calendars. As the Church of the East celebrated the feast of Nativity, it was already 4 January 1583 A.D. on Pope Gregory's new calendar. That gap widened by 3 more days over the next 4 centuries. This is why the Churches who still celebrate on 25 December according to the ancient Julian calendar (such as most of the Byzantine Churches and the non-Chalcedonian churches, except the Armenians) find themselves, in the 21st century, celebrating the Nativity on 7 January of the civil Gregorian new calendar. This will become 8 January after the year 2100 A.D.