Ancient Rome: Months

September comes from the Latin word septem, meaning seven.
October comes from the Latin word octo, meaning eight.
November comes from the Latin word novem, meaning nine.
December comes from the Latin word decem, meaning ten.

Back then, September was the seventh month in their calendar, October was their eighth, November, their ninth, and December had been the tenth month. This was before January and February were added in. March was their first month.