Julius Caesar’s calendar reform helped establish January 1 as the start of the New Year.
The Julian calendar was introduced in 46 BC and took effect in 45 BC. It reorganized the Roman calendar by using a solar year of 365 days, with a leap day added in most years divisible by four. January 1 remained the official start of the consular year, continuing an established Roman political practice.
January was associated with Janus, the Roman god of beginnings, transitions, doorways, and passages. His two-faced imagery made the month especially appropriate for looking backward and forward.
The Julian calendar later accumulated a small error because its average year was slightly too long. Pope Gregory XIII’s Gregorian reform in 1582 corrected that drift, but it retained January 1 as New Year’s Day. Many countries adopted the Gregorian calendar at different times, so calendar changeovers were not simultaneous worldwide.