section_path: Components/Months
section_name: Months
paragraph_text: Months
  
  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 29 or 30 days, with an additional lunar month ("leap month") added periodically to synchronize the twelve lunar cycles with the longer solar year. These extra months are added in seven years (3, 6, 8, 11, 14, 17, and 19) out of a 19-year cycle, known as the Metonic cycle (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 (as is still done in Karaite Judaism and Islam), nowadays Jewish months have generally fixed lengths which approximate the period between new moons. For these reasons, a given month does not always begin on the same day as its astronomical conjunction.
  
  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:
  
  Thus, the year normally contains twelve months with a total of 354 days. In such a year, the month of Marcheshvan has 29 days and Kislev has 30 days. However, due to the Rosh Hashanah postponement rules, in some years Kislev may lose a day to have 29 days, or Marcheshvan may acquire an additional day to have 30 days.
  
  Normally the 12th month is named Adar. During leap years, the 12th and 13th months are named Adar I and Adar II (Hebrew: Adar Aleph and Adar Bet —"first Adar" and "second adar"). Sources disagree as to which of these months is the "real" Adar, and which is the added leap month.