In the early days of Rome there were only 10 months beginning with March. The last six months were given names based on the number of the month so we had SEPTember. OCTOber, NOVember and DECember. Months five and six were eventually renamed for Julius Caesar (July) and Augustus (August). January and February were added later which moved the month numbers away from their original position in the calendar year.

(The Great Courses – Zero to Infinity: A History of Numbers)