Lunisolar structure

The Jewish calendar is lunisolar—i.e., regulated by the positions of both the Moon and the Sun. It consists usually of 12 alternating lunar months of 29 and 30 days each (except for Ḥeshvan and Kislev, which sometimes have either 29 or 30 days) and totals 353, 354, or 355 days per year. The average lunar year (354 days) is adjusted to the solar year (3651/4 days) by the periodic introduction of leap years in order to assure that the major festivals fall in their proper seasons. The leap year consists of an additional 30-day month called First Adar, which always precedes the month of (Second) Adar. A leap year consists of either 383, 384, or 385 days and occurs seven times during every 19-year period (the Metonic cycle). Among the consequences of the lunisolar structure are these: the number of days in a year may vary considerably, from 353 to 385 days; and the first day of a month can fall on any day of the week, that day varying from year to year. Consequently, the days of the week upon which an annual Jewish festival falls vary from year to year despite the festival’s fixed position in the Jewish month.

Months and notable days

The months of the Jewish religious year, their approximate equivalent in the Western Gregorian calendar, and their notable days are as follows:

• Tishri (September–October)

  • 1–2 Rosh Hashana (New Year)
  • 3 Tzom Gedaliahu (Fast of Gedaliah)
  • 10 Yom Kippur (Day of Atonement)
  • 15–21 Sukkoth (Tabernacles)
  • 22 Shemini Atzeret (Eighth Day of the Solemn Assembly)
  • 23 Simḥat Torah (Rejoicing of the Law)
• Ḥeshvan, or Marḥeshvan (October–November)
• Kislev (November–December)

  • 25 Hanukkah (Feast of Dedication) begins
• Ṭevet (December–January)

  • 2–3 Hanukkah ends
  • 10 ʿAsara be-Ṭevet (Fast of Ṭevet 10)
• Shevaṭ (January–February)

  • 15 Ṭu bi-Shevaṭ (15th of Shevaṭ: New Year for Trees)
• Adar (February–March)

  • 13 Taʿanit Esther (Fast of Esther)
  • 14, 15 Purim (Feast of Lots)
• Nisan (March–April)

  • 15–22 Pesaḥ (Passover)
• Iyyar (April–May)

  • 18 Lag ba-ʿOmer (33rd Day of the ʿOmer Counting)
• Sivan (May–June)

  • 6, 7 Shavuot (Feast of Weeks, or Pentecost)
• Tammuz (June–July)

  • 17 Shivaʿ ʿAsar be-Tammuz (Fast of Tammuz 17)
• Av (July–August)

  • 9 Tisha be-Av (Fast of Av 9)
• Elul (August–September)

During leap year the Adar holidays are postponed to Second Adar. Since 1948 many Jewish calendars list Iyyar 5—Israel Independence Day—among the Jewish holidays.

Origin and development

The origin of the Jewish calendar can no longer be accurately traced. Some scholars suggest that a solar year prevailed in ancient Israel, but no convincing proofs have been offered, and it is more likely that a lunisolar calendar similar to that of ancient Babylonia was used. In late Second Temple times (i.e., 1st century bce to 70 ce), calendrical matters were regulated by the Sanhedrin, or council of elders, at Jerusalem. The testimony of two witnesses who had observed the new moon was ordinarily required to proclaim a new month. Leap years were proclaimed by a council of three or more rabbis with the approval of the nasi, or patriarch, of the Sanhedrin. With the decline of the Sanhedrin, calendrical matters were decided by the Palestinian patriarchate (the official heads of the Jewish community under Roman rule). Jewish persecution under the Roman emperor Constantius II (reigned 337–361) and advances in astronomical science led to the gradual replacement of observation by calculation. According to Hai ben Sherira (died 1038), the head of a leading Talmudic academy in Babylonia, the Palestinian patriarch Hillel II introduced a fixed and continuous calendar in 359 ce. A summary of the regulations governing the present calendar is provided by Maimonides, the great medieval philosopher and legist, in his Code: Sanctification of the New Moon, chapters 6–10.

Fragments of writings discovered in a genizah (a depository for sacred writings withdrawn from circulation) have brought to light a calendrical dispute between Aaron ben Meir, a 10th-century Palestinian descendant of the patriarchal (Hillel) family, and the Babylonian Jewish authorities, including Saʿadia ben Joseph, an eminent 9th–10th-century philosopher and gaon (head of a Talmudic academy). Ben Meir’s calculations provided that Passover in 922 be celebrated two days earlier than the date fixed by the normative calendar. After a bitter exchange of letters, the controversy subsided in favour of the Babylonian authorities, whose hegemony in calendrical matters was never again challenged.

Calendars of various sectarian Jewish communities deviated considerably from the normative calendar described above. The Dead Sea, or Qumrān, community (made famous by the discovery of the Dead Sea Scrolls) adopted the calendrical system of the noncanonical books of Jubilees and Enoch, which was essentially a solar calendar. Elements of the same calendar reappear among the Mishawites, a sect founded in the 9th century.

The Karaites, a sect founded in the 8th century, refused, with some exceptions, to recognize the normative fixed calendar and reintroduced observation of the new moon. Leap years were determined by observing the maturation of the barley crop in Palestine. Consequently, Karaites often celebrated the festivals on dates different from those fixed by the rabbis. Later, in medieval times, the Karaites adopted some of the normative calendrical practices while rejecting others.