Hebrew calendar

The Hebrew calendar or Jewish calendar is the calendar used in Judaism. It is used to set the dates of the Jewish holidays and the weekly public reading of the Torah. It is used to set the date for a Bar Mitzvah or Bat Mitzvah, the day a young person is considered an adult in Judaism. It sets the Yahrzeit, the anniversary of the death of a relative. The daily Jewish prayer service changes depending on the day of the Hebrew calendar.

Basic rules
250px|thumb|right|Mosaic floor of a zodiac in the 6th century synagogue at Beit Alpha, Israel.

History
According to the Book of Exodus, Jews have been using a lunisolar calendar since the time they left Egypt. The first commandment the Jewish people received as a nation was the commandment to determine the New Moon. Very soon after that, the Jews received the commandment to make sure that Passover falls in the spring.

In the Tanakh (the Hebrew Bible), the months are usually numbered rather than named. (See table below.) Only four month names appear in the Tanakh from before the Babylonian Exile (see table). The month names of the modern Hebrew calendar were taken from the month names in the Babylonian calendar during the Babylonian Exile (6th century BCE).Ancient Israel: Its Life and Institutions (1961) by Roland De Vaux, John McHugh, Publisher: McGraw–Hill, .

At first, a new month began when witnesses came to the Sanhedrin (high rabbinical court) and testified that they saw the new crescent moon in the sky.The Islamic calendar, which is lunar, still uses a similar procedure today. The Sanhedrin also decided when an extra month should be added to the calendar to keep Passover in the spring. (See Metonic cycle.) After the destruction of the Second Temple in 70 CE, it became harder for the Sanhedrin to meet and hear witnesses. For that reason, the Sanhedrin established a fixed, rule-based form of the calendar. Maimonides fully described the modern Hebrew calendar around the year 1178 CE.The Code of Maimonides (Mishneh Torah), Book Three (Book of "Times" or "Seasons"), Treatise Eight: Laws of the Sanctification of the New Moon. See reference below.

In the modern Hebrew calendar, years are counted as Anno Mundi (Latin for "year of the world"). This represents the traditional count of years since the creation of the world as described in Genesis. This year is anno mundi .

In Karaite Judaism
In the small Karaite Judaism community, the calendar is very similar. However, Karaites use witnesses to set the date of the new crescent moon. They still use the ripening of barley to decide whether to add an extra month to the year. For those reasons, the Karaite calendar can be slightly different from the main Jewish calendar.Karaite Korner calendar web site

Days of the Hebrew calendar week
 Day of the week(Hebrew)  Translation  Day starts fromsundown  Day continues untilsundownיום ראשוןyom rishonfirst daySaturdaySundayיום שניyom shenisecond daySundayMondayיום שלישיyom shelishithird dayMondayTuesdayיום רביעיyom revi'ifourth dayTuesdayWednesdayיום חמישיyom hamishififth dayWednesdayThursdayיום ששיyom shishisixth dayThursdayFridayשבתshabbatShabbat(Sabbath)FridaySaturday

Months
 Month in Bible  Month namein English  Month namein Hebrew  Numberof days  Time of year  Notes 1  Nisan    30  March–April  pre-exile name AvivSee . ("spring"Particularly, aviv means "the [season of the] ripening of barley" ().)Month of Passover 2  Iyar    29  April–May  pre-exile name Ziv ("light") 3  Sivan    30  May–June  Month of Shavuot 4  Tammuz    29  June–July  5  Ab    30  July–August  Month of Tisha B'Av 6  Elul    29  August–September  7  Tishrei    30  September–October  pre-exile name Etanim ("strong")Month of Rosh Hashanah,Yom Kippur and Sukkot 8  Heshvan(or Marheshvan)  מַרְ)חֶשְׁוָן)  29 or 30These can change because of technical calendar rules. See Calculating the calendar above.  October–November  pre-exile name Bul 9  Kislev    30 or 29  November–December  Month of Hanukkah 10  Tevet    29  December–January  11  Shevat    30  January–February  12  Adar    29  February–March  Month of Purim—In a leap year: 12a: Adar I ('אֲדָר א), 30 days 12b: Adar II ('אֲדָר ב), 29 days

Details
The day and the week
In the Hebrew calendar, the meaning of day is taken from the Hebrew Bible:Genesis 1:5 "And there was evening, and there was morning, one day". Because "evening" comes before "morning", a day in the Hebrew calendar starts in the evening. For many purposes, the day in the Hebrew calendar begins at sunset. But when it is important to make sure the previous day is completely over, the day begins at nightfall.

People who print calendars, or who write calendar programs for computers, ignore that rule. They treat the Hebrew calendar date and the Gregorian calendar date that has the same midnight as being the same day. They assume that people reading a Hebrew calendar know the sundown rule. Whenever a holiday, birthday or anniversary appears on a printed calendar, it actually begins on the day before, at sundown.

In the Hebrew calendar, every seventh day is Shabbat, the day of rest.Based on . The week is the cycle of counting out each set of seven days ending in Shabbat. The week is not affected by any other calendar calculation. In Hebrew, the only name for the first six days of the week is a counting name: "First day", "Second day", etc. The only day of the week with a special name is the seventh day—Shabbat.See, for example, Calendar in Hebrew (from he.chabad.org) . (See table above.)

The molad
The calculation of the Hebrew calendar year and month start with the molad. Molad is a Hebrew word meaning "birth". The word refers to the "birth" of the new moon each month. The modern Hebrew calendar uses a calculated molad: the average length of the cycle from new moon to new moon over many years. The length of the molad is 29 days, 12 hours, 44 minutes,  seconds.

Regular years and leap years
A year of 12 lunar months would be 354 days long; but a Hebrew calendar year must be a solar year—about 365 days long. Also, a Hebrew calendar year must have either 12 months or 13 months. It is not allowed to have "part" of a month, so a thirteenth month is added seven times in every cycle of nineteen years. This is an adaptationn 2 of the Metonic cycle, a calendar cycle that was well known in ancient times. By conventionn 3, the Hebrew calendar adds the extra month during years 3, 6, 8, 11, 14, 17 and 19 of the cycle.

Calculating Rosh Hashanah for this year and next year
Hebrew calendar months are supposed to be lunar months. For that reason, the calculation of Rosh Hashanah, the Jewish new year, starts with the molad for the month of Tishrei. Starting from there, the calendar rules allow two main reasons to delay Rosh Hashanah by one or more days: There are actually a total of four postponement rules. The other two are very rare. They are used to make the year the right length in days.
 Postponing Rosh Hashanah if the time of the molad (in Jerusalem) is after 12:00 noon. If not for this rule, Rosh Hashanah would be the day before the molad in parts of the world east of Jerusalem.
 Postponing Rosh Hashanah so that it does not start on Sunday, Wednesday or Friday. This rule prevents Yom Kippur from being the day before or after Shabbat. It also prevents Hoshana Rabba, the seventh day of Sukkot, from being on Shabbat.Once Rosh Hashanah for this year is calculated, the Rosh Hashanah for next year is calculated, starting with the time either 12 or 13(depending on whether it is to be a leap year or not) molad periods in the future.

Once the dates for this year's and next year's Rosh Hashanahs are known, calculating the calendar in between is easy.

Calculating the calendar

The length of a molad is just a little over  days. Since a month must have a whole number of days, calculating the calendar starts with alternating months of 30 and 29 days. This gives an average of  days and a 12-month total of 354 days. Starting from there, the month lengths are changed as follows:
 When the extra month is added, it is month six (counting from Tishrei), and always has 30 days.
 When the number of days between this Rosh Hashanah and next is 355 days in a regular year (or 385 days in a leap year), an extra day is needed. It is added to Heshvan (month two counting from Tishrei), which then has 30 days.
 When the number of days between this Rosh Hashanah and next is 353 days in a regular year (or 383 days in a leap year), one fewer day is needed. It is taken away from Kislev (month three counting from Tishrei), which then has 29 days.
(See the table of months above.)

Calendar months and the astronomical new moon

The first day of the Hebrew calendar month, known as Rosh Hodesh (), is always close to the astronomical new moon. It often is not exactly on the astronomical new moon. There are two reasons for that:
 The calendar calculations are based on the average length of a moon cycle. The actual length of individual moon cycles varies over time.
 The rules for postponing Rosh Hashanah (see above) push Rosh Hodesh away from the astronomical new moon. The people who created the calculations decided having Rosh Hodesh close to the new moon was less important than other factors.

Notes

 References 
 The Code of Maimonides (Mishneh Torah), Book Three, Treatise Eight: Sanctification of the New Moon. Translated by Solomon Gandz. Yale Judaica Series Volume XI, Yale University Press, New Haven, Conn., 1956.
 Bonnie Blackburn and Leofranc Holford-Strevens. The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-reckoning. Oxford University Press; USA, 2000. pp 723–730.

 Other websites 
The English of these sites may not be Simple English.
 Jewish Calendar Details various Jewish points-of-view about the history of the Jewish calendar/Hebrew calendar. Includes several charts.
 Jewish Calendar with Zmanim - Halachic times and date converter chabad.org
 Jewish Encyclopedia: Calendar
 Jewish calendar  More detailed, scientific explanation at the NASA web site
 Calendar Hebrew for Christians website

 Date converters 
 Kaluach - Hebrew/civil calendars 
 Hebcal Hebrew Date Converter
 Chabad Hebrew date converter
 The Kalendis Calendar Calculator  (freeware)

Category:Calendars
Category:Judaism