Originally the Ides were supposed to be determined by the full moon, reflecting the lunar origin of the Roman calendar. Martius (March) was the first month of the Roman year until as late as the mid-second century BC, an order reflected in the numerical names of the months of September (the seventh month) through December (the tenth month) not corresponding to their current position on the Gregorian calendar. In the earliest Roman calendar, the Ides of March would have been the first full moon of the new year. As a fixed point in the month, the Ides accumulated functions set to occur every month, and was the day when debt payments and rents were due.