January 1 was the first day of the year in the Roman calendar after Julius Caesar’s reform.
The Julian calendar was introduced by Julius Caesar in 46 BC and took effect in 45 BC. It reorganized the Roman calendar to create a solar year of 365 days, with a leap day added every fourth year. January 1 remained the beginning of the civil year, continuing a Roman practice that had become established for consular officeholders.
The month January was associated with Janus, the Roman god of beginnings, transitions, doorways, and passages. Its position at the front of the year made the month especially suitable for looking backward and forward. The name “January” comes from Latin Ianuarius, meaning the month of Janus.
The Julian calendar later accumulated a small error because its average year was slightly too long. The Gregorian calendar, introduced in 1582, corrected that drift while retaining January 1 as New Year’s Day. Many countries adopted the Gregorian calendar at different times.