Jewish Calendar
Level: Basic |
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A few years ago, I was in a synagogue, and
I overheard one man ask another, "When is
Chanukkah this year?" The other man smiled
slyly and replied, "Same as always: the 25th of Kislev." This humorous comment
makes an important point: the date of Jewish
holidays does not change from year to year. Holidays are celebrated on
the same day of the Jewish calendar every year, but the Jewish year is not
the same length as a solar year on the Gregorian calendar used by most of
the western world, so the date shifts on the Gregorian calendar.
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 Gregorian calendar used by most of the world has abandoned any correlation
between the moon cycles and the month, arbitrarily setting the length of
months to 28, 30 or 31 days.
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 loses
about 11 days every year and a 13-month lunar gains about 19 days every year.
The months on such a calendar "drift" relative to the solar year. On a 12
lunar month calendar, the month of Nissan, which is supposed to occur in
the Spring, would occur 11 days earlier each year, eventually occurring in
the Winter, the Fall, the Summer, and then the Spring again. To compensate
for this drift, an extra month was occasionally added. The month of Nissan
would occur 11 days earlier for two or three years, and then would jump forward
29 or 30 days, balancing out the drift. In ancient times, this month was
also 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
or Adar Alef. It 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 for Adar are observed,
the one in which a 13-year-old born in Adar becomes a Bar
Mitzvah. 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 are musically inclined, you may find it helpful
to remember this pattern of leap years by reference to the major scale: 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.
In addition, 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. 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.
The year number on the Jewish calendar represents the number of years since
creation, calculated by adding up the ages of people in the Bible back to
the time of creation. However, this does not necessarily mean that the universe
has existed for only 5700 years as we understand years. 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"). For a fascinating (albeit somewhat defensive) article by a nuclear
physicist showing how Einstein's Theory of Relativity sheds light on the
correspondence between the Torah's age of the
universe and the age ascertained by science, see
The
Age of the Universe.
Jews do not generally use the words "A.D." and "B.C." to refer to the years
on the Gregorian calendar. "A.D." means "the year of our L-rd," and we do
not believe Jesus is the L-rd. Instead, we use the abbreviations C.E. (Common
or Christian Era) and B.C.E. (Before the Common Era), which are commonly
used by scholars today.
The "first month" of the Jewish calendar is the month of Nissan, in the spring,
when Passover occurs. However, the
Jewish New Year is in Tishri, the seventh month,
and that is when the year number is increased. This concept of different
starting points for a year is not as strange as it might seem at first glance.
The American "new year" starts in January, but the new "school year" starts
in September, and many businesses have "fiscal years" that start at various
times of the year. Similarly, the Jewish calendar has different starting
points for different purposes.
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. Note
that most of the Bible refers to months by number, not by name.
The Jewish calendar has the following months:
| Hebrew |
English |
Number |
Length |
Gregorian Equivalent |
 |
Nissan |
1 |
30 days |
March-April |
 |
Iyar |
2 |
29 days |
April-May |
 |
Sivan |
3 |
30 days |
May-June |
 |
Tammuz |
4 |
29 days |
June-July |
 |
Av |
5 |
30 days |
July-August |
 |
Elul |
6 |
29 days |
August-September |
 |
Tishri |
7 |
30 days |
September-October |
 |
Cheshvan |
8 |
29 or 30 days |
October-November |
 |
Kislev |
9 |
30 or 29 days |
November-December |
 |
Tevet |
10 |
29 days |
December-January |
 |
Shevat |
11 |
30 days |
January-February |
 |
Adar I (leap years only) |
12 |
30 days |
February-March |
 |
Adar (called Adar II in leap years) |
12 (13 in leap years) |
29 days |
February-March |
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. After
many years of blissful ignorance, I finally sat down and worked out the
mathematics involved, and I have added a page on
The Jewish Calendar: A Closer Look,
which may be of interest to those who want a deeper understanding or who
want to write a Jewish calendar computer program. For the rest of us, there
are plenty of easily accessible computer programs that will calculate the
Jewish calendar for more than a millennium to come. I have provided some
links below.
Note that the number of days between Nissan and Tishri is always the same.
Because of this, the time from the first major festival
(Passover in Nissan) to the last major festival
(Sukkot in Tishri) is always the same.
I maintain a current Jewish calendar on this website.
Unlike most Jewish calendars you will see, my calendar shows the Hebrew months
with the corresponding Gregorian dates.
Most printed Jewish calendars cover a 16-month period: from September of
one year (to include Rosh Hashanah) to December
of the following year. Be aware, however, that some show only the 12-month
period from September to August, and some that claim to have the full 16-month
period show only limited information about September to December of the latter
year. They show the Gregorian months with Jewish
holidays, Torah
readings, candle-lighting times and so forth. I am particularly partial
to the
London
Jewish Museum calendar, which has illustrations of Jewish artwork from
the middle ages to the 1800s, but there are many Jewish calendars available
on Amazon.com.
Click
here to check their catalog.
If you would like to download a Jewish calendar for your computer, I highly
recommend Calendar Maven's Hebrew Calendar, a shareware program that is available
for download at
http://www.calendarmaven.com.
With this program, you can see calendars for dates from the Gregorian year
1600 to the year 2200, including holidays, weekly Torah readings, candle
lighting times and more.
If you would like to look up the date of a Jewish holiday, from the Gregorian
year 1 to the Gregorian year 9999, try
http://www.hebcal.com. I don't know
how accurate this is (especially given that during the earlier dates, months
were determined by observation), but I haven't caught any mistakes in it
yet. Of course, the earlier Gregorian dates are artificial, since the Gregorian
calendar did not exist until the 16th century and was not accepted in many
parts of the world until much later.
If you would like to make your own computerized Jewish calendar, my page
on The Jewish Calendar: A Closer Look
explains in detail how the calendar works and provides illustrative JavaScript
code.
© Copyright 5756-5766 (1995-2005), Tracey
R Rich
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