When did December become the 12th month?

December became the 12th month of the year in 45 BCE with the introduction of the Julian calendar by Julius Caesar. Originally, December was the 10th month in the Roman calendar, which began in March. The Julian calendar restructured the year, placing January and February at the start, thus making December the 12th month. This change has persisted into the modern Gregorian calendar, which was adopted later.