The Julian calendar established January 1 as New Year's Day in the Roman world under Julius Caesar.
Caesar introduced the reform in 46 BC, with the Julian calendar taking effect in 45 BC. The system reorganized the Roman calendar by using a solar year of 365 days and adding a leap day every fourth year. January 1 became the calendar's official starting date, continuing an established Roman administrative practice.
The reform corrected accumulated seasonal drift in the earlier Roman calendar. It was developed with advice from the Alexandrian astronomer Sosigenes, who helped align the calendar more closely with the solar year.
The Gregorian calendar later refined the leap-year rule in 1582, but it retained January 1 as New Year's Day. Thus, the modern date is usually associated with the Gregorian calendar, while the earlier Roman decision about January 1 belongs to the Julian reform.