Which ancient Roman calendar reform helped establish January 1 as the beginning of the civil year?

The Julian calendar reform helped establish January 1 as the beginning of the Roman civil year.

Julius Caesar introduced the reform in 46 BC, and the Julian calendar took effect in 45 BC. The reform regularized the Roman calendar by using a 365-day year with a leap day added every fourth year, correcting problems caused by the older calendar’s irregular intercalation.

January 1 had already become an important Roman administrative date before Caesar’s reform. Consuls began their terms on that day from 153 BC, so the reform reinforced an existing political practice rather than inventing the association from nothing. January’s connection with Janus, the god of beginnings, also gave the date symbolic significance.

The Gregorian calendar, introduced in 1582, corrected the Julian calendar’s small excess in the length of the year. It retained January 1 as New Year’s Day. Thus, the later Gregorian reform changed leap-year accuracy but did not replace the month used to begin the civil year.