Now, the second theory as to why Christmas is celebrated on Dec 25th. May I warn you; it seems very tenuous to us moderns.
In the ancient world few people knew the day they were born, but the day of their death was recorded. As early as the third century, many Christians communities thought March 25 was the day Christ was crucified. This is the day of the spring equinox. March 25 was also considered by many to be the first day of the New Year. For many it was also the day they believed the creation of the world began.
The ancient mind, therefore, creation and redemption were united. The next logical step, for an ancient, was to suggest that Christ died on the day He was conceived, because in this culture, where birthdays were not known, the day of death was always celebrated. In their mind, there was a link between the day of death with the day a person entered the world through conception. To them it seemed natural the day you left the world was the day you entered it. Good Friday, the Annunciation (the day Angel Gabriel announced Jesus conception to Mary), & Christmas therefore all belong together. This combined Jesus incarnation, birth & atonement. Thus, Christmas falls on December 25, nine months after His conception & crucifixion.
Whether this date is true or not, didn’t really matter to the early Christians. What mattered was the truth behind this practice. The incarnation (God becoming a human), and the atonement, (Jesus death for us), belong together. Christmas & Easter are two sides of the same coin. Jesus was born to die. That’s what the Church Year is all about That’s’ what Christmas, Easter & the Annunciation are all about.
Adapted from - Arthur Just: Heaven on Earth; The gift of Christ in the Divine Service, p. 136