It may seem rather strange that some of the names of the months do not match the number of the month. For example, November, the 11th month of the year, means 'nine' and December, the 12th month, means 'ten'. The reason for this is that the Roman calendar originally only had 10 months, beginning with Martius (March). King Numa later increased the number of months to 12 by adding Januarius and Februarius at the end of the year.
When Julius Caesar altered the length of the months in 46 B.C., he also shifted them, resulting in the year beginning with Januarius and ending with Decembris - the original 10th month of the year.