The original Roman calendar (year of Romulus) was a ten-month year based on agriculture. Until 153 BC, March was the first month of the year, and the forms septem-, octo-, novem-, and decem- made a lot more sense in naming the months. Additional changes came with the calendar reform of Julius Caesar in 45 BC.

First of September

Mensis September was
Month number seven when
March was the prime;
Then came the calendar's
Vamp with its changes in
Reckoning time.