Julius Caesar, advised by the astronomer Sosigenes of Alexandria, reformed the chaotic Roman calendar, which had drifted badly out of sync with the seasons due to priestly manipulation. The Julian calendar introduced a 365-day year with a leap day every fourth year, and moved the new year from March to January 1, honoring Janus, the two-faced god of doorways and beginnings.
Before Caesar's reform, the Roman calendar was a mess. Pontiffs were supposed to insert extra months to keep the calendar aligned with the sun, but they often skipped this duty for political reasons—extending the terms of allies or shortening those of rivals. By 46 BC the civil calendar had drifted some three months ahead of the solar year, forcing Caesar to insert 90 extra days into that single year, which Romans grimly nicknamed 'the last year of confusion.'
Sosigenes recommended a solar year of 365.25 days, borrowing Egyptian astronomical knowledge. The choice of January 1 as New Year's Day made practical sense: it was the day Roman consuls took office, a natural point for civic renewal. The god Janus, who looked simultaneously backward and forward, became the fitting patron of the date.
The Julian calendar would govern the Western world for over 1,600 years, until Pope Gregory XIII corrected its slight overcounting of leap years in 1582. Even today, several Orthodox churches still follow the Julian reckoning for religious observances, celebrating Christmas thirteen days after their Gregorian neighbors.