Hebrew calendar
The Hebrew calendar, also called the Jewish calendar, is a lunisolar calendar used today for Jewish religious observance and as an official calendar of Israel. It determines the dates of Jewish holidays and other rituals, such as yahrzeits and the schedule of public Torah readings. In Israel, it is used for religious purposes, provides a time frame for agriculture, and is an official calendar for civil holidays alongside the Gregorian calendar.
Like other lunisolar calendars, the Hebrew calendar consists of months of 29 or 30 days which begin and end at approximately the time of the new moon. As 12 such months comprise a total of just 354 days, an extra lunar month is added every 2 or 3 years so that the long-term average year length closely approximates the actual length of the solar year.
Originally, the beginning of each month was determined based on physical observation of a new moon, while the decision of whether to add the leap month was based on observation of natural agriculture-related events in ancient Israel. Between the years 70 and 1178, these empirical criteria were gradually replaced with a set of mathematical rules. Month length now follows a fixed schedule which is adjusted based on the [|molad interval] and [|several other rules], while leap months are now added in 7 out of every 19 years according to the Metonic cycle.
Nowadays, Hebrew years are generally counted according to the system of Anno Mundi according to traditional Jewish interpretation of the chronology of the Hebrew Bible. This system attempts to calculate the number of years since the creation of the world according to the Genesis creation narrative and subsequent Biblical stories. The current Hebrew year, AM, began at sunset on and will end at sunset on.
Components
Days
Based on the classic rabbinic interpretation of , a day in the rabbinic Hebrew calendar runs from sunset to the next sunset. Similarly, Yom Kippur, Passover, and Shabbat are described in the Bible as lasting "from evening to evening". The days are therefore figured locally.Halachically, the exact time when days begin or end is uncertain: this time could be either sundown or else nightfall. The time between sundown and nightfall is of uncertain status. Thus observance of Shabbat begins before sundown on Friday and ends after nightfall on Saturday, to be sure that Shabbat is not violated no matter when the transition between days occurs.
Instead of the International Date Line convention, there are varying opinions as to where the day changes.
Hours
Judaism uses multiple systems for dividing hours. In one system, the 24-hour day is divided into fixed hours equal to of a day, while each hour is divided into 1080 halakim. A helek is seconds. The ultimate ancestor of the helek was a Babylonian time period called a barleycorn, equal to of a Babylonian time degree. These measures are not generally used for everyday purposes; their best-known use is for calculating and announcing the molad.In another system, the daytime period is divided into 12 relative hours. A relative hour is defined as of the time from sunrise to sunset, or dawn to dusk, as per the two opinions in this regard. Therefore, an hour can be less than 60 minutes in winter, and more than 60 minutes in summer; similarly, the 6th hour ends at solar noon, which generally differs from 12:00. Relative hours are used for the calculation of prayer times ; for example, the Shema must be recited in the first three relative hours of the day.
Neither system is commonly used in ordinary life; rather, the local civil clock is used. This is even the case for ritual times.
Weeks
The Hebrew week is a cycle of seven days, mirroring the seven-day period of the Book of Genesis in which the world is created.The names for the days of the week are simply the day number within the week. The week begins with Day 1 and ends with Shabbat.
Since some calculations use division, a remainder of 0 signifies Saturday.
In Hebrew, these names may be abbreviated using the numerical value of the Hebrew letters, for example יום א׳ :
| Hebrew name | Abbreviation | Translation | English equivalent |
| Yom Rishon | יום א' | First day | Sunset on Saturday to sunset on Sunday |
| Yom Sheni | יום ב' | Second day | Sunset on Sunday to sunset on Monday |
| Yom Shlishi | יום ג' | Third day | Sunset on Monday to sunset on Tuesday |
| Yom Revii | יום ד' | Fourth day | Sunset on Tuesday to sunset on Wednesday |
| Yom Hamishi | יום ה' | Fifth day | Sunset on Wednesday to sunset on Thursday |
| Yom Shishi | יום ו' | Sixth day | Sunset on Thursday to sunset on Friday |
| Yom Shabbat | יום ש' | Sabbath day | Sunset on Friday to sunset on Saturday |
The names of the days of the week are modeled on the seven days mentioned in the Genesis creation account. For example, Genesis 1:8 "... And there was evening and there was morning, a second day" corresponds to Yom Sheni meaning "second day".
The seventh day, Shabbat, as its Hebrew name indicates, is a day of rest in Judaism. In Talmudic Hebrew, the word Shabbat can also mean "week", so that in ritual liturgy a phrase like "Yom Reviʻi beShabbat" means "the fourth day in the week".
Days of week of holidays
Jewish holidays can only fall on the weekdays shown in the following table:The period from 1 Adar to 29 Marcheshvan contains all of the festivals specified in the Bible. The lengths of months in this period are fixed, meaning that the day of week of Passover dictates the day of week of the other Biblical holidays. However, the lengths of the months of Marcheshvan and Kislev can each vary by a day. As a result, the holidays falling after Marcheshvan can fall on multiple days for a given row of the table.
A common mnemonic is "לא אד"ו ראש, ולא בד"ו פסח", meaning: "Rosh HaShana cannot be on Sunday, Wednesday or Friday, and Passover cannot be on Monday, Wednesday or Friday" with each day's numerical equivalent, in gematria, is used, such that א' = 1 = Sunday, and so forth. From this rule, every other date can be calculated by adding weeks and days until that date's possible day of the week can be derived.
Months
The Hebrew calendar is a lunisolar calendar, meaning that months are based on lunar months, but years are based on solar years. The calendar year features twelve lunar months of 29 or 30 days, with an additional lunar month added periodically to synchronize the twelve lunar cycles with the longer solar year. These extra months are added in seven years out of a 19-year cycle, known as the Metonic cycle.The beginning of each Jewish lunar month is based on the appearance of the new moon. Although originally the new lunar crescent had to be observed and certified by witnesses, nowadays Jewish months have generally fixed lengths which approximate the period between new moons. For these reasons, a given month does not always begin on the same day as its astronomical conjunction.
The mean period of the lunar month is very close to 29.5 days. Accordingly, the basic Hebrew calendar year is one of twelve lunar months alternating between 29 and 30 days:
Thus, the year normally contains twelve months with a total of 354 days. In such a year, the month of Marcheshvan has 29 days and Kislev has 30 days. However, due to the Rosh Hashanah postponement rules, in some years Kislev may lose a day to have 29 days, or Marcheshvan may acquire an additional day to have 30 days.
Normally the 12th month is named Adar. During leap years, the 12th and 13th months are named Adar I and Adar II. Sources disagree as to which of these months is the "real" Adar, and which is the added leap month.
Justification for leap months
The Bible does not directly mention the addition of leap months. The insertion of the leap month is based on the requirement that Passover occur at the same time of year as the spring barley harvest. According to the rabbinic calculation, this requirement means that Passover should fall after the March equinox. Similarly, the holidays of Shavuot and Sukkot are presumed by the Torah to fall in specific agricultural seasons.Maimonides, discussing the calendrical rules in his Mishneh Torah, notes:
By how much does the solar year exceed the lunar year? By approximately 11 days. Therefore, whenever this excess accumulates to about 30 days, or a little more or less, one month is added and the particular year is made to consist of 13 months, and this is the so-called embolismic year. For the year could not consist of twelve months plus so-and-so many days, since it is said: "throughout the months of the year", which implies that we should count the year by months and not by days.
Years
New year
The Hebrew calendar year conventionally begins on Rosh Hashanah, the first day of Tishrei. However, the Jewish calendar also defines several additional new years, used for different purposes. The use of multiple starting dates for a year is comparable to different starting dates for civil "calendar years", "tax or fiscal years", "academic years", and so on. The Mishnah identifies four new-year dates:The 1st of Nisan is the new year for kings and festivals. The 1st of Elul is the new year for the cattle tithe, Rabbi Eliezer and Rabbi Shimon say on the first of Tishrei. The 1st of Tishri is the new year for years, of the Shmita and Jubilee years, for planting and for vegetables. The 1st of Shevat is the new year for trees—so the school of Shammai, but the school of Hillel say: On the 15th thereof.
Two of these dates are especially prominent:
- 1 Nisan is the ecclesiastical new year, i.e. the date from which months and festivals are counted. Thus Passover is described in the Torah as falling "in the first month", while Rosh Hashana is described as falling "in the seventh month".
- 1 Tishrei is the civil new year, and the date on which the year number advances. This date is known as Rosh Hashanah. Tishrei marks the end of one agricultural year and the beginning of another, and thus 1 Tishrei is considered the new year for most agriculture-related commandments, including Shmita, Yovel, Maaser Rishon, Maaser Sheni, and Maaser Ani.