The date, in Ancient Rome, when elected officials took office. Until Julius Caesar went to Egypt, the calendar of Ancient Rome was such a mess a long, indefinite period of time marked the year end. Thus, the officer in charge of the calendar could delay what would pass for January 1. This may be why Julius Caesar kept his lowlier calendar duties after he became emperor, but that is just my guess as to his motives. Emperors after Julius followed suit. The calendar instituted after Caesar returned from Egypt is called the Julian calendar.
As Christianity took hold in Rome, emperors ceded more and more of their powers to Popes, which is why Pope Gregory got to set the calendar for the Catholic world after the Julian calendar had gone hopelessly because it was a tad shorter than the time it took the earth to circle the sun. For the Pope, as for the Romans, it was important that certain holidays fall in certain seasons. And that was no longer happening under the Julian calendar.
This was verified by the observations of Clavius, and the new calendar was instituted when Gregory decreed, by the papal bull Inter gravissimas of 24 February 1582, that the day after Thursday, 4 October 1582 would be not Friday, 5 October, but Friday, 15 October 1582. The new calendar duly replaced the Julian calendar, in use since 45 BC, and has since come into universal use. Because of Gregory's involvement, the reformed Julian calendar came to be known as the Gregorian calendar.
The switchover was bitterly opposed by much of the populace, who feared it was an attempt by landlords to cheat them out of a week and a half's rent. However, the Catholic countries of Spain, Portugal, Poland, and Italy complied. France, some states of the Dutch Republic and various Catholic states in Germany and Switzerland (both countries were religiously split) followed suit within a year or two, and Hungary followed in 1587.
However, more than a century passed before Protestant Europe accepted the new calendar. Denmark, the remaining states of the Dutch Republic, and the Protestant states of the Holy Roman Empire and Switzerland adopted the Gregorian reform in 1700–01. By this time, the calendar trailed the seasons by 11 days. Great Britain and its American colonies reformed in 1752, where Wednesday, 2 September 1752 was immediately followed by Thursday, 14 September 1752; they were joined by the last Protestant holdout, Sweden, on 1 March 1753.
The Gregorian calendar was not accepted in eastern Christendom for several hundred years, and then only as the civil calendar.<2> The Gregorian Calendar was instituted in Russia by the Bolsheviks in 1917, Romania accepted it in 1919 under king Ferdinand of Romania (1 November 1919 became 14 November 1919), Turkey in 1923 under Ataturk, and the last Orthodox country to accept the calendar was Greece also in 1923.
While some Eastern Orthodox national churches have accepted the Gregorian calendar dates for feast days that occur on the same date every year, the dates of all movable feasts (such as Easter) are still calculated in the Eastern Orthodox churches by reference to the Julian calendar.
http://en.wikipedia.org/wiki/Pope_Gregory_XIIIAfter adoption of the Gregorian calendar, dates according to the Julian calendar were sometimes designated by an O.S. (for "Old Style") after the date.
The colonies did not adopt the Gregorian calendar until January 1, 1752 (under the Gregorian calendar). Prior to that, they followed the Julian calendar and celebrated New Year's Day on March 25. (Intially, the Gregorian calendar had been considered too Popish for the non-Catholic Brits.)
The switch over was not without hiccups. So many things went into upheaval. Just think of Paul Revere. His birthday was December 21, 1734
O.S., which became January 1, 1735 when the colonies switched to Gregorian. (Things like that have given geneaologists migraines ever since.) Taxes and rents were thrown into chaos, and so on.
http://en.wikipedia.org/wiki/Calendar_%28New_Style%29_Act_1750http://www.crowl.org/lawrence/time/britgreg.htmlJanuary 1 is the date that many nations began their independent existence (as opposed to being colonies).
It is also the anniversary of the Emancipation Proclamation, an Executive Order issued by President Lincoln on January 1, 1863 (151 years ago tomorrow).
Many more important January 1 anniversaries:
http://en.wikipedia.org/wiki/January_1I hope that your January 1 is hangover-free and otherwise wonderful and propitious.