Julius Caesar established January 1 as the start of the year in the Julian calendar.
The Julian calendar was introduced in 46 BC after Caesar ordered a major reform of the Roman calendar. Its first official year began on January 1, 45 BC, after an unusually long transitional year designed to realign the calendar with the seasons.
January had long been associated with the beginning of Roman administrative terms, and its name comes from Janus, the Roman god often represented with two faces looking forward and backward. The calendar reform created a regular 365-day year with a leap day every four years.
The Gregorian calendar later corrected the Julian calendar’s small error in the length of the solar year. Most countries eventually adopted the Gregorian system, but January 1 remained the conventional New Year’s Day. Caesar did not create every New Year custom practiced today; he standardized the calendar framework that made the date central.