Which calendar reform established January 1 as the start of the year in much of the Roman world?

The Julian calendar established January 1 as the start of the year in much of the Roman world.

Julius Caesar introduced the reform in 46 BCE, with the first Julian calendar year beginning on January 1, 45 BCE. The reform replaced a complicated Roman calendar whose months had drifted away from the seasons because of irregular intercalation. The new system used a 365-day year and added a leap day every fourth year.

January 1 had already been important in Roman public life. It was the date when newly elected consuls traditionally entered office, so the reform reinforced an existing administrative year-start rather than creating an entirely unfamiliar custom. The calendar was later adopted across much of Europe.

The Julian calendar slightly overestimated the solar year, accumulating a difference of roughly one day every 128 years. Pope Gregory XIII's Gregorian reform of 1582 corrected that drift. Many countries therefore celebrate January 1 today using the Gregorian calendar, but the Julian reform is the key Roman milestone behind the familiar date.