[yucky]

And the reason for this is because Vitamin K (a major clotting agent) is at its highest levels in humans around the 8th day of their life. Ancient societies didn't know what Vitamin K was, but through trial and error they no doubt discovered that the best day to circumcise a child was therefore on the 8th day of their life.

Since circumcision represents "a new beginning, a new covenant" it made sense for this to begin the New Year. Or conversely, counting back from January 1st it would make sense to have Jesus being born on December 25th.