<P> The Romans did not follow the usual Greek practice in alternating 29 - and 30 - day months and a 29 - or 30 - day intercalary month every other year . Instead, their 3rd, 5th, 7th, and 10th months had 31 days each; all the other months had 29 days except February, which had 28 days for three years and then 29 every fourth year . The total of these months over a 4 - year span differed from the Greeks by 5 days, meaning the Roman intercalary month always had 27 days . Similarly, within each month, the weeks did not vary in the Greek fashion between 7 and 8 days; instead, the full months had two additional days in their first week and the other three weeks of every month ran for 8 days ("nine" by Roman reckoning). Still more unusually, the intercalary month was not placed at the end of the year but within the month of February after the Terminalia on the 23rd (a.d. VII Kal . Mart .); the remaining days of February followed its completion . This seems to have arisen from Roman superstitions concerning the numbering and order of the months . The arrangement of the Roman calendar similarly seems to have arisen from Pythagorean superstitions concerning the luckiness of odd numbers . </P> <P> These Pythagorean - based changes to the Roman calendar were generally credited by the Romans to Numa Pompilius, Romulus's successor and the second of Rome's seven kings, as were the two new months of the calendar . Most sources thought he had established intercalation with the rest of his calendar . Although Livy's Numa instituted a lunar calendar, the author claimed the king had instituted a 19 - year system of intercalation equivalent to the Metonic Cycle centuries before its development by Babylonian and Greek astronomers . Plutarch's account claims he ended the former chaos of the calendar by employing 12 months totaling 354 days--the length of the lunar and Greek years--and biennial intercalary months of 22 days . </P> <P> Plutarch believed Numa was responsible for placing January and February first in the calendar; Ovid states January began as the first month and February the last, with its present order owing to the Decemvirs . W. Warde Fowler believed the Roman priests continued to treat January and February as the last months of the calendar throughout the Republican period . </P> <Table> Pre-Julian Roman calendar <Tr> <Th> English </Th> <Th> Latin </Th> <Th> Meaning </Th> <Th> Length in days </Th> </Tr> <Tr> <Td> January </Td> <Td> Mensis Ianuarius </Td> <Td> Month of Janus </Td> <Td> 29 </Td> </Tr> <Tr> <Td> February </Td> <Td> Mensis Februarius </Td> <Td> Month of the Februa </Td> <Td> 28 </Td> </Tr> <Tr> <Td> Mercedonius Intercalary Month </Td> <Td> Mercedonius Mensis Intercalaris </Td> <Td> Month of Wages </Td> <Td> 23 </Td> </Tr> <Tr> <Td> March </Td> <Td> Mensis Martius </Td> <Td> Month of Mars </Td> <Td> 31 </Td> </Tr> <Tr> <Td> April </Td> <Td> Mensis Aprilis </Td> <Td> Uncertain </Td> <Td> 29 </Td> </Tr> <Tr> <Td> May </Td> <Td> Mensis Maius </Td> <Td> Uncertain </Td> <Td> 31 </Td> </Tr> <Tr> <Td> June </Td> <Td> Mensis Iunius </Td> <Td> Month of Juno </Td> <Td> 29 </Td> </Tr> <Tr> <Td> Quintilis </Td> <Td> Mensis Quintilis Mensis Quinctilis </Td> <Td> Fifth Month </Td> <Td> 31 </Td> </Tr> <Tr> <Td> Sextilis </Td> <Td> Mensis Sextilis </Td> <Td> Sixth Month </Td> <Td> 29 </Td> </Tr> <Tr> <Td> September </Td> <Td> Mensis September </Td> <Td> Seventh Month </Td> <Td> 29 </Td> </Tr> <Tr> <Td> October </Td> <Td> Mensis October </Td> <Td> Eighth Month </Td> <Td> 31 </Td> </Tr> <Tr> <Td> November </Td> <Td> Mensis November </Td> <Td> Ninth Month </Td> <Td> 29 </Td> </Tr> <Tr> <Td> December </Td> <Td> Mensis December </Td> <Td> Tenth Month </Td> <Td> 29 </Td> </Tr> </Table>

When were january and february added to the calendar
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