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The Hebrew calendar (Hebrew: הלוח העברי‎) or Jewish calendar is the calendar used by Jews for religious purposes. It is a lunisolar calendar used to reckon the Jewish New Year and to determine the dates for Jewish holidays, the appropriate Torah portions for public reading, Yahrzeits (dates to commemorate the death of a relative), and which daily Psalm is to be read, among many ceremonial uses. Originally the Hebrew calendar was used by Jews for all quotidian purposes, but by the era of the Roman occupation (1st Century BCE), Jews were compelled to follow the imperial civil calendar for all civic matters such as the payment of taxes and dealings with government officials.

The Hebrew calendar's epoch (reference date), 1 Tishrei 1 anno mundi, is equivalent to Monday, October 7, 3761 BCE in the proleptic Julian calendar, the equivalent tabular date (same daylight period) and is about one year before the traditional Jewish date of Creation on 25 Elul AM 1, based upon the Seder Olam Rabbah of Rabbi Yossi ben Halafta, a second century CE sage.[1] Thus, adding 3760 or 3761[2] to any Julian/Gregorian year number after 1 CE will yield the Hebrew year. For earlier years there may be a discrepancy (see: "Missing Years" in the Hebrew Calendar).

Two major forms of the calendar have been used. Before the destruction of the Second Temple in 70 CE, the calendar was observational, with the beginning of each month determined by the testimony of witnesses who had observed a new crescent moon. Between 70 and 1178 CE a rule-based fixed-arithmetic lunisolar calendar system was adopted to achieve the same effect.

The origins of the Hebrew calendar are found in the Torah, which refers to the existence of several numbered but un-named months in the Noahide (pre-Jewish) period, and which recounts several calendar-based commandments, including God's commandment during the Exodus from Egypt to fix the month of Nisan as the first month of the year.[3] The development of the calendar was likely influenced by the Babylonian exile in the 6th Century BCE, during which Babylonian names for the months were adapted; the Babylonians also employed a lunisolar calendar derived from the Sumerian calendar. Following the Jewish diaspora of Roman times (c. 1st Century CE), calculations were increasingly used to fix dates, with the principles fully described by Maimonides in 1178 CE in the Mishneh Torah.
Because of the roughly eleven-day difference between twelve lunar months and one solar year, the year lengths of the Hebrew calendar vary in a repeating 19-year Metonic cycle of 235 lunar months, with an intercalary lunar month added every two or three years, for a total of 7 times per 19 years. Seasonal references in the Hebrew calendar reflect its development in the region east of the Mediterranean Sea and the times and climate of the Northern Hemisphere. With respect to the present-day mean solar year, the Hebrew calendar's year is longer by about 6 minutes and 25+25/57 seconds, meaning that every 224 years, the Hebrew calendar will fall a full day behind the modern fixed solar year, and about every 231 years it will fall a full day behind the Gregorian calendar year. This is due to the 0.6 second discrepancy between the Calendric "Molad" (lunar conjunction interval), which is fixed by Jewish Law,[4] and the actual mean lunar conjunction interval, which itself is slowly changing over time.[5]


Biblical period

Mosaic pavement of a zodiac in the 6th century synagogue at Beit Alpha, Israel. Biblical references to the pre-Jewish calendar include ten months identified by number rather than by name. In parts of the Torah portion Noach (Noah) (specifically, Genesis 7:11, 8:4, :5, :13, :14) it is implied that the months are thirty days long.[6] There is no indication as to the total number of months in the annual cycle.

In the parts of the Tanakh (the Hebrew Bible) prior to the Babylonian exile, only four months are named: Aviv (first; literally "Spring", originally this probably meant "the ripening of barley"), Ziv (second; literally "Light"), Ethanim (seventh; literally "Strong" in plural, perhaps referring to strong rains), and Bul (eighth). All of these are Canaanite names, and at least two are Phoenician (Northern Canaanite).

The Torah includes a number of commandments related to the keeping of the calendar and the lunar cycle, including the first commandment the Jewish people received as a nation, to determine the new moon: Exodus 12:2 states, "This month [Nissan] is for you the first of months." Numbers 10:10 refers to the observation of the new moon, "Also in the day of your gladness, and in your appointed seasons, and in your new moons, ye shall blow with the trumpets over your burnt-offerings,"[7] and Numbers 28:11 commands, "And in your new moons ye shall present a burnt-offering unto the LORD: two young bullocks, and one ram, seven he-lambs of the first year without blemish."[8] Deuteronomy 16:1 refers to a specific month: "Observe the month of Abib, and keep the passover unto the LORD thy God; for in the month of Abib the LORD thy God brought thee forth out of Egypt by night."[9]

However, the adoption of a purely lunar calendar cycle would have resulted in the starting date of the first month of the lunar year progressively drifting away from the Spring, due to the difference in length between twelve lunar months and a solar year. Since the Torah specifies not only the significance of the lunar month, but also mandates that the dates of certain festivals correspond to a given season of the year, it is to be inferred that a system of reconciling the lunar months and the solar year was in use. The Bible does not directly mention the addition of an "embolismic" or intercalary month that would prevent the drifting of the calendar year, however, it is hinted that the first month was started only following the ripening of barley, and according to some traditions, if the barley had not yet ripened, another monthly cycle would be interjected after the "last month" (Adar), before the first month of the year was announced.

Thus, if Adar was over and the barley was not yet ripe, an additional month, "Adar II" was observed before Nisan. During such leap years Adar I (or Adar Aleph "first Adar") is actually considered to be the extra month, and has 30 days. Adar II (or Adar Bet "second Adar") is the "real" Adar, and has the usual 29 days. For this reason, during a leap year, holidays such as Purim are observed in Adar II, not Adar I. Later, a mathematical system was developed to replace the observational method.

Babylonian exile and nomenclature

Detail of the Ishtar Gate of Babylon, dating from the era of the Babylonian captivity.During the Babylonian exile, which started in 586 BCE, Jews adopted Babylonian names for the months, which have continued to be used to this day. The Babylonian calendar also used a lunisolar calendar, derived from the Sumerian calendar. The Hebrew calendar follows the common seven-day weekly cycle, with the Hebrew names for the weekdays simply the day number within the week. In Hebrew, these names may be abbreviated using the numerical value of the Hebrew letters, for example יום א׳ (Day 1, or Yom Rishon (Hebrew: יום ראשון):

Yom Rishon (Hebrew: יום ראשון), abbreviated יום א׳ = "first day" = Sunday
Yom Sheni (יום שני), abbr. יום ב׳ = "second day" = Monday
Yom Shlishi (יום שלישי), abbr. יום ג׳ = "third day" = Tuesday
Yom Reviʻi (יום רבעי), abbr. יום ד׳ = "fourth day" = Wednesday
Yom Ḥamishi (יום חמישי), abbr. יום ה׳ = "fifth day" = Thursday
Yom Shishi (יום ששי), abbr. יום ו׳ = "sixth day" = Friday
Yom Shabbat (יום שבת or more usually שבת - Shabbat), abbr. יום ש׳ = "Sabbath day (Rest day)" = Saturday

In Hebrew, the word "Shabbat" (שַׁבָּת) can also mean "(Talmudic) week",[10] so that in ritual liturgy a phrase like "Yom Reviʻi bəShabbat" means "the fourth day in the week".[11]

Hebrew names and romanized transliteration may somewhat differ, as they do for כסלו / Kislev or חשוון / Marheshvan: the Hebrew words shown here are those commonly indicated e.g. in newspapers.

Hebrew names of the months with their Babylonian analogs
Number Hebrew Academy Common/Other Length Babylonian analog Notes
1 נִיסָן Nisan Nissan 30 days Nisanu called Aviv and Nisan in the Tanakh
2 אִיָּר / אייר Iyyar Iyar 29 days Ayaru called Ziv in the Tanakh
3 סִיוָן / סיוון Siwan Sivan 30 days Simanu  
4 תַּמּוּז Tammuz Tamuz 29 days Du'uzu  
5 אָב Av Ab 30 days Abu  
6 אֱלוּל Elul Elul 29 days Ululu  
7 תִּשׁרִי Tishri Tishrei 30 days Tashritu called Eitanim in the Tanakh.
Modern first month, Rosh Hashana is celebrated in Tishrei.
8 מַרְחֶשְׁוָן / מרחשוון Marẖeshwan Marcheshvan 29 or 30 days Arakhsamna often shortened to Cheshvan; called Bul in the Tanakh
9 כִּסְלֵו / כסלוו Kislew Kislev, Chisleu 30 or 29 days Kislimu also spelled Chislev
10 טֵבֵת Tevet Tebeth 29 days Tebetu  
11 שְׁבָט Shevat Shvat, Shebat 30 days Shabatu  
12* אֲדָר א׳ Adar I*   30 days Adaru *Only in leap years
12 / 13* אדר / אדר ב׳ Adar / Adar II*   29 days

Second Temple era, c. 518 BCE - 70 CE