Happy Arbitrary New Year
At our New Year's Eve party we got onto the topic why Jan. 1st Christian Era is the New Year in the Western part of the world. My personal speculation, unsupported by any evidence or historical research, is that it is around the Winter Solstice time. If you live really far North, the difference in day length is more pronounced, but as you go farther South, where these determinations of the Holidays were made, the change is day length is more subtle, slower to notice.
I think the real scholars with a sense of history and astronomy, were able to pinpoint the shortest day of the year pretty closely, even before atomic clocks and all the electrically driven devices we now have. The average isolated village in olden days though, probably were more likely to have depended on sense perception to tell when the days were getting longer. The 25th of December is about when a good observer would have been able to know for sure the days were longer. Which would be why the Saturnalia and Christmas were celebrated then , "Whew, the Sun is coming back, we have avoided total destruction for another year, let's give thanks."
For the more dimly aware, perhaps because of sleeping late due to overindulgence, probably weren't able to tell the difference until about January 1st, and responded in their usual manner, "Whoo hooo, we shall live another year, let's get hammered and celebrate."
All of which still begs the question of why the New Year is celebrated more or less at the Solstice time. In China for instance, it is more like late February, or thereabouts, when the new life of spring is stirring. I believe Rosh Hasannah is the Jewish New Year which is like in September, around the fall Equinox. Even in the traditions that use the name Krishna for God, the New Year is considered to be at other times. I think that the Hindu New Year is Holi, which could be consider the harvest festival time, the end of the growing season and the beginning of anticipation for the next. For Gaudiya Vaisnavas, it is Lord Chaitanya's Appearance day, though in practice, no one really observes or thinks of it as such, as far as I can tell. That is around the Spring Equinox.
So I guess you can pretty much take your pick :-) (All opinions expressed in this post are poorly researched and subject to error.)