According to the Julian calendar, tonight is the eve of the new year. Most of the world uses the Gregorian calendar, but it was used for over 16 centuries. (The Christian Eastern Orthodox Church still uses the Julian calendar.)
The Gregorian calendar was accepted in 1582 as being more accurate and it eventually replaced the Julian though it didn’t happen overnight or even worldwide in that year. Astronomers still use the Julian calendar dates for celestial events occurring before the Gregorian calendar was introduced.
The Julian calendar had discrepancies between the calendar dates and the actual time of events like the spring equinox. It was Pope Gregory who decreed that October 4, 1582, on the Julian calendar was to be followed by October 15, 1582, in the new Gregorian calendar. England, with its own church, stuck with the Julian calendar for two more centuries.
The Julian calendar was proposed by Julius Caesar in AUC 708 (46 BC). It was a more accurate version of the existing Roman calendar and it took effect on 1 January AUC 709 (45 BC), by his edict. It was designed with the aid of Greek mathematicians and Greek astronomers for accuracy and was the predominant calendar in the Roman world and most of Europe for more than 1,600 years.