The Jewish calendar is based 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, so there is no direct correlation between them. On average, the moon revolves around the Earth in about 29½ days. The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.
The civil calendar used internationally, specially in the western world, does not correlate the moon cycles and the month, arbitrarily setting the length of months to 28 days (February in non-leap years), 29 days (February in leap years), 30 days (April, June. September, and November) or 31 days (all the rest - January, March, May, July, August, October, December).
The Jewish calendar, however, 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.
The lunar month on the Jewish calendar begins when the first sliver of moon becomes visible after the dark of the moon. In ancient times, the new months used to be determined by observation. 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 (first of the month) and send out messengers to tell people when the month began.
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 leap month). 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 (Passover) would occur in the spring (it is, after all, referred to in the Torah as Chag he-Aviv, the Festival of Spring!).
A year with 13 months is referred to in Hebrew as Shanah Me'uberet (pronounced shah-NAH meh-oo-BEH-reht), literally: a pregnant year. In English, we commonly call it a leap year. The additional month is known as Adar I, Adar Rishon (first Adar) or Adar Alef (the Hebrew letter Alef being 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). Note that Adar II is the "real" Adar, the one in which Purim is celebrated, the one in which yahrzeits (anniversaries in Yiddish, used for death anniversaries) for Adar are observed, the one in which a 13-year-old born in Adar becomes a Bar Mitzvah. Thus Adar I is the "extra" Adar.
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). If you know the arrangement of piano black and white keys, you may find it helpful to remember this pattern of leap years by reference to the piano keyboard where going from one white key to the next adjacent white is considered a whole step, and from a white to adjacent black a half step: for each whole step there are two regular years and a leap year; for each half-step there is one regular year and a leap year. This is easier to understand when you examine the keyboard illustration below and see how it relates to the leap years above.
The current year is 5772 which commenced on 1st Tishri (28th September 2011). Since the current cycle began in Jewish year 5758 (the year that began October 2, 1997), the current year is the 14th year of the cycle which is completed in 29th Ellul 5776 (2nd October 2016).
Some additional considerations in fixing length of months:
Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah should not fall on Saturday because it would interfere with the holiday's observances. So 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. If you are interested in the details of how these calculations are performed, see Detailed Calendar Calculations.
If you want to know where we are today on the Jewish Calendar, go to the online calendar of aish.com, The default view is the calendar for New York City, including exact times of the start and end of Shabbat, candle lighting times, etc. A dropdown list of world cities enables you to select your city, or any other city of interest, for instance, Jerusalem, Israel. Feasts are in beige, today's date in ochre, and the rest of the dates in light blue.
- Yom Rishon (the first day)
- Yom Sheni (the second day)
- Yom Shlishi (the third day)
- Yom Revi'i (the fourth day)
- Yom Chamishi (the fifth day)
- Yom Shishi (the sixth day)
- Yom Shabbat, more usually called simply Shabbat (Sabbath Day, which means Rest Day)
1st of the month - New Moon - Rosh Hodesh (Literallly, Head of the month)
14th of he month - Full Moon (there are feasts that fall on the full moon - Pesach and the first day of Succoth)
Yearly Cycle of Months - Biblical Calendar
- The first month, Aviv (Spring) also called Nissan - the first month of Jewish Kings and Biblical Festivals
- The second month, Iyar
- The third month, Sivan
- The fourth month, Tammuz
- The fifth month, Av
- The sixth month, Elul - the first month for the tithing of cattle
- The seventh month, Tishri - the first month of the civil calendar and also for counting years of Gentile Kings.
- The eighth month, Cheshwan / Marcheshwan
- The ninth month, Kislev
- The tenth month, Tevet
- The eleventh month, Shevat - the first month for tithing of trees, the New Year of Trees celebrated today on the 15th Shevat.
- The twelfth month, Adar
Yearly Cycle of Months - Civil
- Tishri - starts with Rosh Hashanah, the civil New Year
- Cheshwan / Marcheshwan
- Shevat The month in which the New Year of Trees is celebrated.
- Adar (in certain specified years, defined by the 19-year cycle, an leap month is added after Shevat and is called Adar I, the next month, called Adar II, is treated as the "real" Adar. Thus Purim is celebrated in Adar II in leap years.)
- Aviv / Nissan The month in which Passover is celebrated, hence the first month of the calendar of Feasts.
- Ellul The first month of the year for tithing of cattle.
See the 4 Jewish New Years
Years / Epoch
The Jewish calendar uses the Anno Mundi epoch, Year of the World, counting from the year of creation, based on chronological data in the BIble. This is abbreviated to AM or A.M. According to Rabbinic tradition, the year 1 is not the year of creation, but one year before creation. The first day of the year is 1 Tishri 1 Anno Mundi, equivalent in the proleptic Julian calendar to 7 October 3761 BCE).
The Hebrew Year 5771 (year before the current year) began on 9 September 2010, and since it is a leap year, will end on 28 September 2011 (29 Elul).
19-Year Cycle (Metonic Cycle)
This is a cycle of 19 years, of which 12 are common years of 12 months, and 7 are leap years of 13 months.
I intend to add more detail concerning this topic of the Metonic Cycle and the topic of the Epoch briefly touched on above.
Cycle of Appointments with the Lord (Mo'edim) (that's the Biblical term for them, since they are appointments to travel to Jerusalem to observe the "feast")
- Pentecost, also known as Feast of Weeks (Shavuot)
- Feast if Trumpets, Day of Blowing (Yom Teruah)
- Day of Atonement (Yom Kippur)
- Feast of Tabernacles (Sukkot)
Cycle of Fasts
- 17th Tammuz (The Fast of the Fourth Month) The walls of Jerusalem were breached in 69 CE after a long siege by the Roman Army. The Jerusalem Talmud states that the Babylonians breached the walls of Jerusalem before destroying the First Temple, on this day, 423 BCE.
- Tisha b'Av - Ninth of Av (The Fast of the Fifth Month) The First Temple was destroyed in 423 BCE by Nebuchadnezzar. The Second Temple was destroyed in 69 CE by General Titus of the Roman army. The city of Betar was conquered by the Romans in 133 CE (which ended the Bar Kochba rebellion), the city was pillaged and hundreds of thousands of Jews were slaughtered. The Temple Mount was ploughed exactly a year after the conquest of Betar. The Jews were expelled from England in 1290. The Jews were expelled from Spain in 1492. World War I was declared on the 9th of Av in 1914.
- 7th of Adar, the date set by Jewish tradition to commemorate the Death of Moses. Many Jews fast during the day, and hold a banquet after sundown, to celebrate Moses' life!
- The Fast of Esther was a 3-day fast, undertaken by Esther to prepare herself to risk her life by going into the presence of the King unsummoned, knowing that for any inmate of the harem to go to the king without being summoned could result in her instand execution. The Fast of Esther is one in which the Jews commemorate the bravery of Queen Esther, and is observed on Adar 13. The full story is told in the Book of Esther, which is read every year on the 14th of Adar at the Festival of Purim
- Yom Kippur (The Fast of the Seventh Month). This was mandated in the Torah, in Leviticus 23.
- 10th Tevet: Asarah b'Tevet (The Fast of the Eleventh Month) This was the date that the Babylonian armies laid siege to Jerusalem. Thirty months later, in 423 BCE, the city walls were breached, and on the 9th of Av of the same year the Holy Temple was destroyed by fire.
There are also larger cycles such as the Sabbatical Year Cycle, the Jubilee Cycle, the 490 year Cycle. See links below.
I am studying the links and the evidence now, and will write up my own conclusions on this site when done.
The Jewish calendar from the Christian Messianic view point.