It seems they started out with 10 (probably lunar) months, starting with March. The period which would later be January and February, probably was more or less dedicated to hibernation ;-)

The first four months had proper names, the other six were numbered. Later, January and February were added. Long after the year start was shifted to January, Cesar occupied the first of the (now wrongly) numbered months (5th = Quintilis became Iulius).