Julius Caesar established January 1 as the start of the year in the Julian calendar.
Caesar introduced the Julian calendar in 46 BCE, with the reformed system taking effect in 45 BCE. The reform addressed accumulated drift in the older Roman calendar, whose months and leap adjustments no longer tracked the solar year reliably.
January was associated with Janus, the Roman god depicted with two faces looking forward and backward. That symbolism made the month a fitting beginning for a civic calendar, and January 1 had already held importance for Roman political officeholders before the reform.
The Julian calendar was later replaced for civil purposes by the Gregorian calendar, introduced in 1582 under Pope Gregory XIII. The Gregorian system retained January 1 as New Year’s Day while correcting the Julian calendar’s small but cumulative error in the length of the year.