Originally, new year was the start of the northern hemisphere spring. If you don’t believe me, examine the names of the months. September, October, November and December mean the 7th, 8th, 9th and 10th months respectively. So some counting on your fingers, and you’ll see that March was the 1st month.
In 365 AD, the Emperor Constantine organised a synod of all the religious leaders he could gather, and they met for a year. Their combined work is what we now know as the Bible. They did their best, given the sources available to them, although they were careful to exclude some documents that didn’t agree with Constantine’s aims.
One of their tasks was to make the new religion of Christianity attractive to that other popular eastern religion, Mithraism. One of its great attractions was the Midwinter Fest, celebrating winter solstice, which fell on 25th December on the Julian calendar. It involved fun activities like slaughtering a bull.
The Synod decided to celebrate Jesus’ birthday on this date. In this, no doubt they copied the British, who celebrate the reigning monarch’s birthday on the day Queen Victoria was born.
The learned members of the Synod knew that Jesus was a Jew, and that in Jewish tradition, a person’s life was considered to start on the day of circumcision, 7 days after the birth.
Some more counting on your fingers: what is 25 + 7?
And there you have it.
I wish you a preposterous new ear.