Martin of Braga (A.D. 510/520-579, Spanish bishop and saint, on why January 1 is not the beginning of the new year:
Likewise this error holds ignorant and rustic men, that they think that January 1 is the beginning of the year. This is altogether most false. For, as Sacred Scripture says, the beginning of the year was established on March 25 at the time of the equinox. For thus is it written: and God divided the light from the darkness (Genesis 1:4). For all right division has equality, as on March 25 the day has the same length of hours as the night. And therefore it is false that the beginning of the year is on January 1. (De correctione rusticorum, trans. D. Herlihy, in Medieval Culture and Society)
March 25, of course, is also Annunciation Day, the day on which the angel Gabriel allegedly announced to Mary that she was pregnant with the Savior, exactly nine months before the traditional date of his birth on December 25. As readers of Tolkien are also aware, March 25 is (not coincidentally) the date on which the Ring was destroyed in Mount Doom, marking the end of the Third and beginning of the Fourth Age (the previous December 25, by comparison, is the date on which the Fellowship set out from Rivendell).