Jews borrowed important aspects of their lunar-solar calendar from the Babylonians.
In some contexts, Nisan is considered the first month of the Jewish calendar. In other contexts, Tishri is considered the first month. For example, Rosh Ha-Shannah, the Jewish New Year, begins the month of Tishri.
| Babylonian | Jewish | Solar events |
|
1. Nisanu 2. Airu 3. Simanu 4. Duzu 5. Abu 6. Ululu |
7. Nisan 8. Iyyar 9. Sivan 10. Tammuz 11. Ab 12. Elul |
Spring Equinox |
|
7. Tashritu 8. Arasamnu 9. Kislinu 10. Tebetu 11. Shabatu 12. Addaru |
1. Tishri 2. Cheshvan 3. Kislev 4. Tebeth 5. Shebat 6. Adar |
Fall Equinox |
| A second Ululu or Addaru was added as necessary as an intercalary month. |
A second Adar is added as necessary as an intercalary month. |
Somewhere betwen 500 and 380 BCE, the Persians noticed that adding seven intercalary months every 19 years kept the lunar calendar from straying from the solar calendar. (Cf. Metonic cycle).