The number of days in each month varies. This list suggests the number of days typically found in each month yet is not conclusive.
Nissan 1 is the head of the year. It occurs at the sighting of the new moon (first sliver crescent) after Barley ripens in the Aviv (pronounced “Aveev”) season or literally when the Barley is in the Aviv stage.
Month 1 (Nissan)
30 Days
March – April
Month 2 (Iyar)
29 Days
April – May
Month 3 (Sivan)
30 Days
May – June
Month 4 (Tammuz)
29 Days
June – July
Month 5 (Av)
30 Days
July – August
Month 6 (Elul)
29 Days
August – September
Month 7 (Tishri)
30 Days
September – October
Month 8 (Cheshvan – Pronounced Heshvan)
29 or 30 Days
October – November
Month 9 (Kislev)
30 or 29 Days
November – December
Month 10 (Tevet)
29 Days
December – January
Month 11 (Sh’vat)
30 Days
January – February
Month 12 (Adar I – leap years only)
30 Days
February – March
Month 13 (Adar II – in leap years)
29 Days
February – March
During a Jewish leap year, an extra month is added after the month of Shevat and before the month of Adar. It is called Adar Aleph, Adar Rishon, or Adar I. The month of Adar is then referred to as Adar Bet, Adar Sheni, or Adar II. A leap year occurs 7 times during the 19-year Metonic cycle, namely, in years 3, 6, 8, 11, 14, 17, and 19 of the cycle. What this means is that a leap month is added every 2 to 3 years.
The Metonic cycle is also known as Enneadecaeteris which is a period that comes very close to being a common multiple of the solar year and the lunar month (synodic month). With a deviation of just a few hours, 19 solar years have very nearly the same length as 235 lunar months, both periods amount to 6940 days.
This makes it possible for Jewish time reckoning to approximately stay in synchronization with the solar year simply by adding a certain amount of full months per Metonic cycle. Since 19 years with 12 months add up to 228 months, 7 extra months must be added to arrive at a total of 235 months per cycle.