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

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