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 December

Mensis December was
Last in a year having
Ten months in all;
Calendars now, though, are
Leaving that prefix for
"Ten" in a stall.