<Tr> <Td> December </Td> <Td> Mensis December </Td> <Td> Tenth Month </Td> <Td> 29 </Td> </Tr> <P> The consuls' terms of office were not always a modern calendar year, but ordinary consuls were elected or appointed annually . The traditional list of Roman consuls used by the Romans to date their years began in 509 BC . </P> <P> Gnaeus Flavius, a secretary to the pontifex maximus, introduced a series of reforms in 304 BC . Their exact nature is uncertain, although he is thought to have begun the custom of publishing the calendar in advance of the month, depriving the priests of some of their power but allowing for a more consistent calendar for official business . </P> <P> Julius Caesar, following his victory in his civil war and in his role as pontifex maximus, ordered a reformation of the calendar in 46 BC . This was undertaken by a group of scholars apparently including the Alexandrian Sosigenes and the Roman M. Flavius . Its main lines involved the insertion of ten additional days throughout the calendar and regular intercalation of a single leap day every fourth year in order to bring the Roman calendar into close agreement with the solar year . The year 46 BC was the last of the old system and included 3 intercalary months, the first inserted in February and two more--Intercalaris Prior and Posterior--before the kalends of December . </P>

When did the months change from 10 to 12