The Jewish Calendar: An Overview
A few years ago, I was in a synagogue, and I overheard one man ask
another, "When is Channukah 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 primarily lunar, with each month beginning
on the new moon, 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 month calendar, the month of Nissan, which
is supposed to occur in the Spring, occurs 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: a second month of Adar. 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 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 II is added in the 3rd, 6th, 8th, 11th, 14th,
17th and 19th years of the cycle. The new year that began Monday, September
25, 1995 (Jewish calendar year 5756) was the 18th year of the cycle.
Jewish year 5758 (beginning October 2, 1997) will be the first year
of the next cycle.
In addition, Yom Kippur should not fall
adjacent to a Sabbath, because this would
cause difficulties in coordinating the fast with the Sabbath, and Hoshanah
Rabba should not fall on Saturday because it would interfere with
the holiday's observances. A day is added to the month of Heshvan or
subtracted from the month of Kislev of the previous year to prevent
these things from happening.
The year number on the Jewish calendar represents the number of years
since creation, as calculated by adding up the ages of people in the
Bible back to the time of creation. However, it is important to note
that this date is not necessarily supposed to represent a scientific
fact. For example, many Orthodox Jews will readily acknowledge that
the seven "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").
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).
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 Jewish calendar has the following months:
||29 or 30 days
||30 or 29 days
||29 or 30 days
In leap years, Adar has 30 days. In non-leap years, Adar has 29 days.
The length of Heshvan 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.
I won't pretend to understand the mathematics involved, and I don't
particularly recommend trying to figure it out. There are plenty of
easily accessible computer programs that will calculate the Jewish calendar
for more than a millennium to come.
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.
Source: Judaism 101