<P> The Hebrew calendar is a lunisolar calendar, meaning that months are based on lunar months, but years are based on solar years . The calendar year features twelve lunar months of twenty - nine or thirty days, with an intercalary lunar month added periodically to synchronize the twelve lunar cycles with the longer solar year . (These extra months are added seven times every nineteen years . See Leap months, below .) The beginning of each Jewish lunar month is based on the appearance of the new moon . Although originally the new lunar crescent had to be observed and certified by witnesses, the moment of the true new moon is now approximated arithmetically as the molad, which is the mean new moon to a precision of one part . </P> <P> The mean period of the lunar month (precisely, the synodic month) is very close to 29.5 days . Accordingly, the basic Hebrew calendar year is one of twelve lunar months alternating between 29 and 30 days: </P> <Table> <Tr> <Th> No . </Th> <Th> Hebrew months </Th> <Th> Length </Th> </Tr> <Tr> <Td> </Td> <Td> Nisan </Td> <Td> 30 </Td> </Tr> <Tr> <Td> </Td> <Td> Iyar </Td> <Td> 29 </Td> </Tr> <Tr> <Td> </Td> <Td> Sivan </Td> <Td> 30 </Td> </Tr> <Tr> <Td> </Td> <Td> Tammuz </Td> <Td> 29 </Td> </Tr> <Tr> <Td> 5 </Td> <Td> Av </Td> <Td> 30 </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Elul </Td> <Td> 29 </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Tishrei </Td> <Td> 30 </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Marcheshvan (or Cheshvan) </Td> <Td> 29 / 30 </Td> </Tr> <Tr> <Td> 9 </Td> <Td> Kislev </Td> <Td> 30 / 29 </Td> </Tr> <Tr> <Td> 10 </Td> <Td> Tevet </Td> <Td> 29 </Td> </Tr> <Tr> <Td> 11 </Td> <Td> Shevat </Td> <Td> 30 </Td> </Tr> <Tr> <Td> 12 </Td> <Td> Adar </Td> <Td> 29 </Td> </Tr> <Tr> <Th_colspan="2"> Total </Th> <Td> 353, 354 or 355 </Td> </Tr> </Table> <Tr> <Th> No . </Th> <Th> Hebrew months </Th> <Th> Length </Th> </Tr>

What is the twelfth month of the jewish calendar