[rsynnott]

AIUI, there is some confusion over whether this is actually the case. The pre-Julian calendar had 12 months, plus an optional intercalated month (they were aware that their ‘year’ had the wrong number of days, and periodically shoved in some extra time to patch it up). The 10 month calendar, if it existed, would have been very early and there’s not much hard evidence that it was actually used. Numa Pompilius, who was allegedly responsible, is a mythical figure and probably not an actual historical king.