The modern Hebrew or Jewish Calendar came into use about the 4th century AD (or CE meaning Common Era). It was determined by calculation whereas the previous calendar since the Exodus had been determined by observation of the phasis of the moon, that is the first appearance of the crescent new moon. This would not be visible until at least a day after the true astronomical new moon, depending on the timing of that new moon and the weather conditions.

The modern Jewish Calendar was probably first implemented by the Patriarch Hillel II in 358/359 AD, but until the 10th century AD there was disagreement about the proper years for intercalation and the initial epoch (starting date) for reckoning years.

Lunisolar

The modern Hebrew Calendar is complicated because the calendar months have to be strictly lunar, but the seasonal feasts must appear in their proper solar seasons. The calendar has to harmonise simultaneously with both lunar and solar events, and so is described as lunisolar. Maimonedes once said,

"The method of the fixed calendar is one which an average school child can master in 3 or 4 days"� (Hilchot Kiddush Hachodesh 11:4)!

Month

As far as we know month names were first used in Israel's lunisolar calendar after the Exodus, but they underwent some changes after the Babylonian captivity and modern usage is slightly different again...

  1. Nisan 30 days
  2. Iyyar 29 days
  3. Sivan 30 days
  4. Tammuz 29 days
  5. Av 30 days
  6. Elul 29 days
  7. Tishri 30 days
  8. Heshvan 29 or 30 days
  9. Kislev 29 or 30 days
  10. Teveth 29 days
  11. Shevat 30 days
  12. Adar I - Intercalary 30 days
  13. or 13. Adar II 29 days

Rules

Number of months

There are 12 months in a common year and 13 in a leap year. The final month is always Adar II - the interpolated 12th month of 30 days in a leap year is Adar I to distinguish it from the final month Adar II which then becomes the 13th month.

The ordering of the months follows Biblical convention (Leviticus 23:5) in which Nisan is the first month of the year. However, the Jewish Civil New Year Rosh HaShanah is on the first of Tishri, the 7th month.

Leap years

Leap years occur in the 3rd, 6th, 8th,11th, 14th, 17th and 19th years of the 19-year Metonic cycle that is named after the Greek astronomer Meton who in 432 BCE published his observation that 19 solar years contain almost exactly 235 months. 19 years of 12 months = 228 months, so an extra 7 months have to be added or intercalated every 19 years to give 235 months. The Metonic cycle is also used in the Julian and Gregorian Calendars to calculate Easter.

Length of year

The start of the Jewish new year is determined by the mean conjunction of the new moon at the beginning of the 7th month, Tishri, but is subject to a possible postponement of one or two days if any of four possible delays occurs. The four Dehiyyot (postponement rules) are as follows...

  • Dehiyyah Molad Zakein. This is the "Old Moon" or "Obsolete Moon" rule. The new year is delayed if the mean conjunction is 18 hours or more after sunset. The periods between sunset (6 hours) and sunrise (18 hours), and sunrise (18 hours) and sunset (6 hours), are each divided into 12 hours so that the actual length of an hour varies between day-time and night-time and according to the season. The conjunction ends up unaffected postponement actually occurs if the mean conjunction is at midday or after, that is 18 hours or more after sunset.
  • Dehiyyah Adu. The name ADU is an acronym formed by the Hebrew letters alef (=1 for Sunday), daled (=4 for Wednesday) and vov (=6 for Friday). The new year must not be on Sunday, Wednesday or Friday. Excluding Wednesday and Friday prevents Yom Kippur (Tishri 10, the Day of Atonement) from falling on Friday or Sunday, and excluding Sunday prevents Hoshana Rabba (Tishri 21) from falling on Saturday (Shabbat). If the conjunction occurs on Saturday , Tuesday or Thursday afternoon this rule combines with Dehiyyah Molad Zakein to cause a two day delay.
  • Dehiyyah GaTaRad. Because of the irregular effect of Dehiyyah Adu, an additional delay is occasionally needed to keep the length of the year within the permitted ranges. If the new year conjunction occurred before noon on Tuesday in a common year and the conjunction of the following year at noon or later, then applying the previous two delays would in turn delay the following Rosh HaShanah from Saturday or Sunday, the day of the next conjunction for a common year, until Monday. This would make the year 356 days long, so instead the current Rosh HaShanah is delayed until Thursday giving a year of 354 days. The prior year cannot become too long because of this delay since its new year conjunction must have been on Friday in a common year or Wednesday in a leap year and would have been delayed for a day by Dehiyyah Adu. This rule is not found in the Talmud.
  • Dehiyyah BeTU'TeKaPoT. Rosh HaShanah on Monday after a leap year can pose a similar problem by causing the year just ending to be too short when the prior new year conjunction was after noon on Tuesday and so was delayed until Thursday. If the conjunction occurred after noon on Tuesday the previous year, in the current year it would occur after 9:32:431/3 am on Monday. In this case, Rosh HaShanah is postponed from Monday to Tuesday, extending the leap year just ending from 382 days to 383. This rule is not found in the Talmud. A normal year has 353 - 355 days, whereas a leap year has 383 - 385 days.

Day

The day is traditionally divided into 24 hours and the hour is divided into 1,080 parts (halaqim), making 25,920 parts in a day. (1,080 is a multiple of 360 and the division into 1,080 parts stems from the Babylonian use of 360 for a time (a Babylonian year) of 360 days and the division of the circle into 360 degrees.) The periods between sunset (6 hours) and sunrise (18 hours), and sunrise (18 hours) and sunset (6 hours), daylight hours and night-time hours, are each divided into 12 hours so that the actual length of an hour varies between day-time and night-time and according to the season.

Other dates

Rosh Hodesh

Rosh Hodesh is the New Month or New Moon.

If the previous month has 29 days, then Rosh Hodesh only lasts for one day. If the previous month has 30 days, then the thirtieth day is celebrated as the first day of Rosh Hodesh and the first day of the new month is celebrated as the second day of Rosh Hodesh. The beginning of the month is not affected if Rosh Hodesh is observed for 2 days.

The mean lunar conjunction or time of the new moon is called the Molad. The period of the Molad is worked out from calendar computations to within 1 day in 14,000 years as 29 days, 12 hours and 793 parts.

Passover

The Passover starts at the opening of 15 Nisan. It lasts eight days (seven in Israel), commemorating the Exodus from Egypt and the freeing of the children of Israel from slavery. The first two and last two days (in Israel the first and seventh days) are full holidays, and the remaining intermediate days, Hol Hamoed, are half holidays. It is one of the three pilgrim festivals (along with Shavuot and Succot) on which pilgrims were supposed to go to worship in the Temple in Jerusalem.

Passover incorporates the Feast of Unleavened Bread, during which unleavened bread, Matzah, is eaten.

Yom Hashoah

This is Holocaust Memorial Day and is celebrated on 27 Nisan. It was introduced in 1950 to commemorate the six million Jews who died in the Holocaust.

Omer

The seven weeks following the first day of Passover are called the Omer, or heave offering, that was the offering of new barley thought to have been brought to the Temple on the second day of Passover. The 49 days of Omer celebrate the 7 weeks of the grain harvest between Passover and Shavuot.

Shavuot

Shavuot is the 50th day after Passover and is celebrated on Sivan 6. It is one of the three pilgrim festivals (along with Passover and Succot) on which pilgrims were supposed to go to worship in the Temple in Jerusalem. In modern Jewish tradition it is celebrated as The Feast of Weeks because it falls seven weeks after Passover and The Festival of Firstfruits, and it commemorates the giving of the Torah on Mount Sinai.

It is observed for two days (one in Israel).

Yom Hazikaron

Yom Hazikaron, Iyar 4, is Israel Memorial Day commemorating those who died in Israel's wars of liberation and defence.

This fast day is moved forward to the previous Wednesday if it would otherwise have occurred on a Thursday or Friday. It is always the day before Yom Ha'Atzmaut.

Yom Ha'Atzmaut

Yom Ha'Atzmaut, Iyar 5, is Israel Independence Day. It was introduced in 1948.

This fast day is moved forward to the previous Thursday when it would otherwise have occurred on a Friday or Saturday. It is always the day after Yom Hazikaron.

Pesach Sheini

Pesach Sheini, Iyar 14, is observed to mark the period of one month's grace available to anyone who could not deliver the Paschal lamb at Passover.

Lag B'omer

The seven weeks following the first day of Passover are called the Omer, or heave offering, that was the offering of new barley thought to have been brought to the Temple on the second day of Passover. The 49 days of Omer celebrate the 7 weeks of the grain harvest between Passover and Shavuot.

Lag B'Omer, Iyar 18, is the 33rd day of Omer. Lag represents the combined gematria or numerical value of two Hebrew letters, lamed = 30, and gimel = 3. On this day the plague that had decimated Rabbi Akiva's disciples during the Second Century CE came to an end. Because of the tragic events of this period, partial mourning is observed although marriages can be performed and other mourning observances are relaxed.

Yom Yerushalayim

Yom Yerushalayim, Iyar 28, is Jerusalem Unification Day commemorating the recapture of the Old City of Jerusalem from the Jordanians during the Six Day War in 1967.

Tzom Tammuz 17

Tzom (the fast of) Tammuz 17 is a day of fasting commemorating the breach made in the defence wall surrounding Jerusalem in 586 BCE. Three weeks later this led to the capture of the city and the destruction of the Temple by the Babylonians under Nebuchadnezzar, and then to the exile in Babylon.

This fast day is postponed to the following day (Sunday) when it would otherwise have occurred on a Saturday.

Tisha B'av

Tisha B'av, Av 9, is a day of fasting commemorating the destruction of the First and Second Temples. This fast day is postponed to the following day (Sunday) when it would otherwise have occurred on a Saturday. Ironically, this rule applies on Av 9 3830, the supposed date of the destruction of the Second Temple and one of the two dates the festival is meant to commemorate!

Rosh Hashanah

Rosh Hashanah, Tishri 1, is the Jewish New Year and is observed for two days throughout Israel and the Diaspora. It marks the commencement of the Ten Days of Penitence which reach their climax at Yom Kippur. The shofar (ram's horn) is sounded on Rosh Hashanah.

It is also known as Yom Hadin, the day of judgement. Tradition holds that on this day God judges mankind for his actions during the past year.

Tzom Gedaliah

Tzom (the fast of) Gedaliah, Tishri 3, is observed in memory of Gedaliah ben Ahikam, the Jewish governor of Judea appointed by Nebuchadnezzar to govern the conquered Judean territory during the Babylonian captivity. Gedaliah did much for those Jews who remained in Judah, and when he was assassinated on Tishri 3 Nebuchadnezzar exiled more of the surviving population to Babylon, destroying the last Jewish hopes for a peaceful restoration.

This fast day is postponed to the following day (Sunday) when it would otherwise have occurred on a Saturday.

Yom Kippur

Yom Kippur, Tishri 10, is the Day of Atonement. It is the holiest day of the Jewish Calendar. It is marked by fasting and prayer and is the day to ask forgiveness from man and God and to offer restitution for any wrongdoing. Tradition holds this day seals one's fate for the coming year.

Sukkot

Sukkot, Tishri 15, is the Feast of Tabernacles or Booths (Hebrew sukkot). It is one of the three pilgrim festivals (along with Passover and Shavuot) on which pilgrims were supposed to go to worship in the Temple in Jerusalem. Its name is derived from the sukkah or booth that Jews build and spend time in during the holiday to commemorate God's protection during the 40 years in the wilderness.

The first two days (in Israel the first day) are full holidays. The following five intermediate days (six in Israel), Hol Hamoed, are partial holidays.

Sukkot is also a harvest festival since it is the time of ingathering the fruit crop in Israel. Throughout the Sukkot festival four species (citron, myrtle, palm, and willow) are carried around the synagogue during parts of the service.

Hoshanah Rabbah (Jewish Calendar)

Hoshanah Rabbah, Tishri 21, is a half holiday on the seventh day of Sukkot. It is celebrated by the recitation of many hosha-na prayers ("help us, we pray"). These are recited by worshippers carrying bundles of willow twigs as they make seven circuits of the synagogue according to the tradition that on Hoshana Rabbah God judges the amount of rainfall needed to insure bountiful harvests.

Shemini Atzeret

Shemini Atzeret, Tishri 22, is the Eighth Day of Solemn Assembly, observed the day after Sukkot. In Israel, Shemini Atzeret also marks the completion of the annual cycle of Torah reading (see Simchat Torah) in the Diaspora.

Simchat Torah

Simchat Torah, Tishri 23 in the Diaspora or Tishri 22 in Israel, is the holiday of Rejoicing in the Torah that starts the annual cycle of reading the Torah (Pentateuch).

Hanukkah

Hanukkah, Kislev 25, is the Festival of Dedication or Festival of Lights. It commemorates the victory of the Maccabees over the Syrians in 165 BCE and the re-dedication of the Temple in Jerusalem that the Syrians had defiled. An additional candle is lit on each night of the festival until on the last night eight candles are burning. According to tradition, when the Temple was recaptured only one small vessel of consecrated oil was found. This was enough for just one day, but miraculously it sufficed for eight days until new oil could be prepared and the festival was established for this period to commemorate the miracle.

Tzom Tevet

Tzom Tevet (the fast of Tevet 10) commemorates the beginning of the siege of Jerusalem by Nebuchadnezzar (586 BCE) which was the prelude to the destruction of the First Temple and the Babylonian exile of the Jews. This fast day is postponed to the following day (Sunday) when it would otherwise have occurred on a Saturday.

Tu B'shevat

Tu B'shevat ("Tu"), Shevat 15, is made up of the two Hebrew letters Tet (9) and Vav (6) which have the combined the gematria or numerical value of 15. In the Talmud it is called the New Year of the Trees because it is the date from which the tithing year of fruit trees is calculated. In contemporary Israel it is marked by the planting of trees and is often called the Jewish Arbor Day. It is customary to eat the fruits of Israel on Tu B'Shevat.

Purim Katan

Purim Katan, Adar I 14, is observed in a leap year to duplicate the precepts of the observance of Purim on Adar ll 14. Since Purim and Passover should be observed close together, Purim Katan is only a partial observance without fasting.

Ta'anith Esther

Ta'anith (the fast of) Esther, Adar 13, commemorates Esther's fasting before pleading with King Ahasuerus to spare the Jewish people (see Purim).

This fast day is moved forward to the previous Thursday when it would otherwise have occurred on a Sunday.

Purim

Purim, Adar 14, commemorates the deliverance of Persian Jews from destruction by Haman, the chief minister of King Ahasuerus (circa fifth century BCE). The holiday is marked by reading from the Megillah, the Book of Esther, that tells the story of how Queen Esther and Mordecai managed to foil Haman's plot.

Purim is the joyful occasion for parties and other festivities. It is customary to send food gifts (mishloach manot) to friends and to distribute charity to the poor. A special festive meal (purim seudah) is held on Purim afternoon.

Shushan Purim

Shushan Purim, Adar 15, is the day on which Purim is celebrated in Jerusalem and the other cities that had been walled cities since the days of Joshua because at the time of the events of Purim, the Jews of Shushan were given an extra day to defeat their enemies.

This day is postponed to the following day (Sunday) when it would otherwise have occurred on a Saturday.