| The Jewish New Year is in Tishri when the years advance in number. However, Nissan is regarded as the first month (incorporating Passover) and happens in the spring, and yet the months are lunar. Without adjustment, an 11 day creep backwards through the solar calendar (as in Islam) would prevent a spring Nissan for many years. So the Jewish calndar is based on both the lunar cycle and the solar cycle. The month begins immediately after a dark moon gives way to part of it being lit. A 12-month lunar calendar loses about 11 days every year and a 13-month lunar gains about 19 days. Just as the Sanhedrin originally called the beginning of the month with moonlight reappearing according to two witnesses, so they added an extra month to bring spring back into line for Passover along with the solar year (which, of course, determines the seasons). A 13 month year is called a pregnant year or Shanah Me'uberet. Adar I, Adar Rishon or Adar Alef is placed before the regular month of Adar (becoming Adar II, Adar Sheini or Adar Beit) but the regular month remains the location for Purim. Hillel II (in 4th century CE) used mathematical and astronomical calculations to make the added month a regular feature based a 19 year cycle. Adar I appears in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years. The current cycle began October 2, 1997 in the Jewish year 5758 (a year number decided by a backwards calculation of biblical events, something of a passion to creationists but not Orthodox Jews as such). A day gets added to Cheshvan or removed from Kislev of the previous year to avoid Yom Kippur becoming adjacent to Shabbat (with fasting difficulties) and stopping Hoshanah Rabbah happening on Saturday. Month names are Babylonian in origin. |
| Month name | Length |
| Nisan (March–April) | 30 |
| Iyar (April–May) | 29 |
| Sivan (May–June) | 30 |
| Tammuz (June–July) | 29 |
| Av (July–August) | 30 |
| Elul (August–September) | 29 |
| Tishri (September–October) | 30 |
| Cheshvan (October–November) | 29 - in some 30 |
| Kislev (November–December) | 29 - in some 30 |
| Tevet (December–January) | 29 |
| Shevat (January–February) | 30 |
|
Adar Sheni Adar (February–March) |
29 (exists in leap year only) 29 - in some 30 |
Adrian Worsfold