On the Jewish Calendar

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How do you calculate the Jewish calendar

The Jewish calendar is based on the old correlations between the moon cycles and the month and on three astronomical phenomena: the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year). These three phenomena are independent of each other, but the Jewish calendar coordinates all three of these astronomical phenomena. Months are either 29 or 30 days, corresponding to the 29½-day lunar cycle. Years are either 12 or 13 months, corresponding to the 12.4 month solar cycle.

Rosh Chodesh

The lunar month on the Jewish calendar begins when the first sliver of moon becomes visible after the dark of the moon, which in ancient times, used to be determined by direct observation. According to the Book of Exodus: "And G-d spoke unto Moses and Aaron in the land of Egypt, saying: 'This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.'" (12:1-2) When people observed the new moon, they would notify the Sanhedrin. When the Sanhedrin heard testimony from two independent, reliable eyewitnesses that the new moon occurred on a certain date, they would declare the Rosh Chodesh (ראש חודש, the head or the beginning of the month) and send out messengers to tell people when the month began. According to the Book of Numbers, G-d speaks of the celebration of the new moon to Moses: "And on your joyous occasions - your fixed festivals and new moon days - you shall sound the trumpets over your burnt offerings and your sacrifices of well-being." (10:10)

Despite the existence of a fixed calendar, Rosh Chodesh is still announced in synagogues on the preceding Shabbat (called Shabbat Mevarchim--The Shabbat of Blessing). The announcement is made during the morning service, after the reading of the Sefer Torah (ספר תורה), before returning it to the Aron Kodesh (the Torah ark), in the prayer of Birkat HaChodesh, mentioned by the medieval sages, beginning "May it be Your will... that You renew this month for us for good and for blessing." The name of the new month, and the day of the week on which it falls, is given during the prayer. Some communities customarily precede the prayer by an announcement of the exact date and time of the new moon, referred to as the molad, or "birth." The incoming month is blessed and G-d is beseech for granting the blessing and protection during the following month. Rosh Chodesh Tishrei (which is also Rosh Hashana) is never announced. On the Shabbat directly preceding the month of Tishrei we do not bless the incoming month during the Shabbat prayers as is traditionally done on the Shabbat before a new month. This is unnecessary for the month of Tishrei, when every Jew is well aware of the impending holiday of Rosh Hashanah which signals the entry of the new year as well as the new month. In addition, we cannot bless the incoming Rosh Chodesh as we do each month, because the first day of Tishrei is not called Rosh Chodesh, but Rosh Hashanah.

During the evening service of Rosh Chodesh, a prayer Ya'a'le Ve-Yavo is added to the Avodah, the prayer for the restoration of the Temple and a segment of the Amidah (תפילת העמידה, Tefilat HaAmidah "The Standing Prayer", or Shmoneh Esreh(18)/שמנה עשרה, in reference to the original number of constituent blessings, the central prayer). During the morning service, Ya'a' le Ve-Yavo is again recited and either a whole or half Hallel (Psalms 113-118) is recited, and is read The Book of Numbers 28:1-15, which includes the offerings of Rosh Chodesh. An additional prayer service, called Mussaf, is added to commemorate the original sacrifices in the Temple. After the service, many recite Psalm 104. The Ya'a'le Ve-Yavo prayer is also inserted in the Grace after Meals ( ברכת המז,Birkat Ha-Mazon). Many have a custom to make sure to eat a special meal in honor of Rosh Chodesh, as the Code of Jewish Law suggests. Some Hasidic Jews sing Psalm 104 during this meal.

If Rosh Chodesh falls on Shabbat, the regular Torah reading is supplemented with a reading of Numbers 28:9-16. The Mussaf prayer is also modified when Rosh Chodesh falls on Shabbat. The central benediction is replaced with an alternate version (Ata Yatzarta) that mentions both the Shabbat and Rosh Chodesh. If Rosh Chodesh falls on a Sunday, a different Haftarah, Mahar Hodesh (I Samuel 20:18-42) is read. The Kiddush Levanah (sanctification of the moon) is recited soon after Rosh Chodesh, typically on the first Saturday night after Rosh Chodesh.

According to the Talmud (tractate Megillah 22b), women are exempt from work on Rosh Chodesh, and Rashi, in commenting on this passage, delineates the activities from which they may refrain: spinning, weaving, and sewing — the skills that women contributed to the building of the Mishkan (Tabernacle). The midrash Pirke De-Rabbi Eliezer explores this exemption in chapter 45:"Aaron argued with himself, saying: If I say to Israel, Give ye to me gold and silver, they will bring it immediately; but behold I will say to them, Give ye to me the earrings of your wives and of your sons, and forthwith the matter will fail, as it is said, "And Aaron said to them, Break off the golden rings." The women heard (this), but they were unwilling to give their earrings to their husbands; but they said to them: Ye desire to make a graven image of a molten image without any power in it to deliver. The Holy One, blessed be He, gave the women their reward in this world and the world to come. What reward did He give them in this world? That they should observe the New Moons more stringently than the men, and what reward will He give them in the world to come? They are destined to be renewed like the New Moons, as it is said: Who satisfieth thy years with good things; so that thy youth is renewed like the eagle."

Female-centered Rosh Chodesh observances vary from group to group, but many are centered on small gatherings of women, called Rosh Chodesh groups. These groups engage in a wide variety of activities that center around issues important to Jewish women, depending on the preference of the group's members, exploring spirituality, religious education, ritual, health issues, music, chanting, art, and/or cooking. Some groups also choose to educate young Jewish women in their community about sexuality, self-image, and other women's mental and physical health issues.

Drifting around the seasons

The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar is about 11 days shorter than a solar year and a 13-month lunar is about 19 longer than a solar year. The months drift around the seasons on such a calendar: on a 12-month lunar calendar, the month of Nissan, which is supposed to occur in the Spring, would occur 11 days earlier in the season each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again. On a 13-month lunar calendar, the same thing would happen in the other direction, and faster.

To compensate for this drift, the Jewish calendar uses a 12-month lunar calendar with an extra month occasionally added. The month of Nissan occurs 11 days earlier each year for two or three years, and then jumps forward 30 days, balancing out the drift. In ancient times, this month was added by observation: the Sanhedrin observed the conditions of the weather, the crops and the livestock, and if these were not sufficiently advanced to be considered "spring," then the Sanhedrin inserted an additional month into the calendar to make sure that Pesach would occur in the spring (as being referred to in the Torah as Chag he-Aviv, the Festival of Spring). At a later date, a custom was developed in which an additional day could be added to the month to ensure that certain holidays (such as Yom Kippur) did not fall on the days before or after Shabbat.

A year with 13 months is referred to in Hebrew as Shanah Me'uberet (literally: a pregnant year). The additional month is known as Adar I, Adar Rishon (first Adar) or Adar Alef (as, according the letter Alef is the numeral "1" in Hebrew). The extra month is inserted before the regular month of Adar (known in such years as Adar II, Adar Sheini or Adar Beit). Adar II is the "real" Adar, the one in which Purim is celebrated, the one in which yahrzeits for Adar are observed, the one in which a 13-year-old born in Adar becomes a Bar Mitzvah, while Adar I is the "extra" Adar. The current year, 5771, began on 9 September 2010 and ends on 28 September 2011.

In the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The current cycle began in Jewish year 5758 (the year that began October 2, 1997).

Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah (the last day of Sukkot. According to the Zohar, while the judgment for the new year is sealed on Yom Kippur, it is not delivered until the end of Sukkot, during which time one can still alter their verdict and decree for the new year. The common blessing exchange during this is פסקא טבא /piska tava), or, in Yiddish, "A guten kvitel"/"A good note") should not fall on Saturday because it would interfere with the holiday's observances. A day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening. This process is sometimes referred to as "fixing" Rosh Hashanah.

Numbering of Jewish Years

The year number on the Jewish calendar are considered the number of years since creation, calculated by adding up the ages of people in the Tanach back to the time of creation. Many Orthodox Jews will readily acknowledge that the first six "days" of creation are not necessarily 24-hour days (indeed, a 24-hour day would be meaningless until the creation of the sun on the fourth "day").

Months of the Jewish Year

The "first month" of the Jewish calendar is the month of Nissan, in the spring, when Pessach occurs. However, the New Year is in Tishri, the seventh month, and that is when the year number is increased.

The names of the months of the Jewish calendar were adopted during the time of Ezra, after the return from the Babylonian exile. The names are actually Babylonian month names, brought back to Israel by the returning exiles: Nissan, Iyar, Sivan, Tamuz, Av, Elul, Tishrei, Cheshvan, Kislev, Tevet, Shevat, Adar I (Adar II).

The length of Cheshvan and Kislev are determined by complex calculations involving the time of day of the full moon of the following year's Tishri and the day of the week that Tishri would occur in the following year. The number of days between Nissan and Tishri is always the same. Because of this, the time from the first major festival (Pessah in Nissan) to the last major festival (Sukkot in Tishri) is always the same.

Tishrei is the month of the beginning and coincides with the September-October in the civil calendar. It is the month when you have Rosh Hashanah, Tzom Gedalia, Yom Kippur, Sukkot, Shemini Atzeret and Simchat Torah. Cheshvan is without any special holiday and occurs in October-November in the usual calendar. Kislev which falls in November-December is the month when we celebrate Hanukkah. Hanukkah continues with Tevet (who starts today) who falls in December-January. Shevat is covering January-February. Its most important event is Tu B’Shvat. Adar is the month of Purim. Nisan is the month of spring, when we celebrate Pessach, Yom Hashoah and start the counting of the Omer. Iyar falls in April-May in the civil calendar and it is dedicated to Yom Hazikaron, Yom Haazmaut, Lag B’Omer and Yom Yerushalaym. Sivan falls in May-June and this month we celebrate Shavuot. Tammuz falls in June-July and is the month of the Tzom Tammuz. Av is the only month mentioned in the Torah and covers July and August, with the day of Tisha B’Av. On Elul, occurring in August-September, we don’t have holiday, because preparing for Rosh Hashanah and Yom Kippur. All month we sound the shofar to wake us up and remind us that the high holidays are coming up.