How did December become the twelfth month?

December became the twelfth month of the year due to its origins in the Roman calendar, which initially began in March. The name "December" comes from the Latin Word "decem," meaning ten, reflecting its position as the tenth month in that calendar. In 713 BCE, January and February were added to the beginning of the year, shifting December to the twelfth position. This change was solidified when the Julian calendar was adopted in 45 BCE, establishing the twelve-month structure we recognize today.