Who introduced January 1 as the start of the year in the Julian calendar?

Julius Caesar introduced January 1 as the start of the year in the Julian calendar.

Caesar's calendar reform took effect in 45 BCE, after the Roman calendar had accumulated serious inconsistencies. The Julian calendar used a 365-day year with a leap day every fourth year, creating a more regular system than the calendar it replaced.

January 1 had already been an important Roman civic date before the reform, especially because Roman consuls traditionally began their terms on that day. Caesar's reform helped establish the date as the calendar year's formal beginning across the Roman system.

Pope Gregory XIII later introduced the Gregorian calendar in 1582 to correct the Julian calendar's gradual seasonal drift. The Gregorian reform changed the leap-year rule and the date used by many countries, but it did not originate the Julian calendar's January 1 year opening.