Good question. A few things first. When Jesus was born, 25 December as a date in time did not exist. At the time of Christ, the common calendar used in the Roman Empire was a variation of the solar calendar that the Romans borrowed and modified from the Greeks, 13-month cycle of 28-30 days each, depending on the relative position of the Sun. What we have today, 365 days, 12 month, 24 hour days was not adopted in the West until the time of Constantine in the middle of the 4th century AD and was based on lunar cycles.
In the first three centuries of the church, the “Nativity of the Lord”, or a day of Christian remembrance and feast, was not openly or formally celebrated due to outlaw status the faith had in the Roman Empire at the time. As an underground movement each local faith community more or less choose their own dates or a time of recognition. By the time of Constantine in the middle of the 4th century, and the legalization of the Church in the Empire, the annual time of celebration and remembrance of the birth of Christ was more or less forming around what was the once celebrated as the Festival of Apollo in the old Pagan empire. The last week of December in the new calendar. As the Christian church moved out of Rome and spread in the old Empire, many of the Christian Holidays we know today were set on the same days or times of former Pagan deities.
Back to the question on whether Jesus was really born on the day we know as 25 December. No one really knows. The Gospel writers took great time and effort to set the birth in a general historical context of the real world. In Luke we read about the 15th year of Tiberius, 4BC in our time, Herod of Galilee, and the priesthood of Caiaphas, all confirmed in secular history within 2 years of the same date, Jesus was born. The Gospel writers did not either do not concern themselves, nor did they see the exact day of the week or date in the Julian calendar. In fact, to this day the Easter Orthodox Church does not recognize 25 December as the day of “Christmas”. Their Holiday comes two weeks later, 6 January. 25 December was not “officially” set as the day of celebration in the Catholic Church until the early 5th century.
I would not get too worked up about the exact day of the birth of Christ in our 2010 calendars. The core fact is that in real history God became flesh in Jesus Christ, in real history he grew up and fed 5000 and raised Lazarus from the dead. In real history, he carried his own Cross and rose again on the third day, and in real history He will come again to call His people home. That is to be celebrated, not just on 25 December, but every day on our calendar.
— Pastor Gregory