The calendar in common use throughout the western
world is based on the sun. Neither the year nor the months have anything to
do with the phases of the moon. The Muslims, on the other hand, reckon both
the year and the month according to the phases of the moon. Their year is therefore
shorter than the general year by about eleven days. A moon-year has 354 days,
and a sun-year 365 days. The Jewish calendar is based on a compromise between
the two, and is reckoned according to both the sun and the moon. The months
are figured according to the moon (twelve months of 29.5 days each), and the
year according to the sun. In order to take up the extra eleven days, a whole
month is added to the calendar in leap years. Every second or third year there
is a thirteenth month, a second Adar.
The Jewish calendar is a very old one. It has been established a long time and
every point and detail has been ironed out. But the history of the calendar,
how it evolved and how, in time, it came to be an established fact, is very
It is to be presumed that in pre-historic times, when Jews were still nomadic
shepherd tribes in the wilderness, they reckoned time entirely by the moon,
as did all nomadic peoples. But it seems that after they settled in Palestine
and began to observe the agriculture seasons, they also began to reckon according
to the position of the sun. How the Jews of the period equalized the sun-year
and the moon-year we do not know. It is possible that at one time they just
added a number of days at the end of each year. In time, however, the method
of making every second or third year a leap year was apparently established.
It appears that in the old days Jews figured their calendar - the month, the
year, and the festivals - entirely by observation, by testimony offered that
the moon had appeared and had been seen. Later, astronomic calculation was instituted
in connection with the calendar, but the Jews were not certain of its exactness
and still had recourse to witnesses. The authority to hear this testimony and
through it to establish the beginning of the month, the intercalation of the
calendar, and the dates of the festivals was vested in the Sanhedrin.
When they accepted the report of the witnesses, the New Moon
was announced through the lighting of fires on the hill-tops. Later, this method
was not considered safe enough, and messengers were sent out to proclaim the
date. However, it took time for the messengers of the Sanhedrin in Palestine
to reach further lands inhabited by Jews and proclaim there the arrival of the
New Moon. It was, therefore, decreed that outside of Palestine, in the lands
of the Diaspora, festivals were to be observed for two days instead of one.
This added second day was called "the second holiday of the Diaspora."
An exception was made in the case of Yom Kippur which, because of the hardship
of fasting, could not be prolonged. Rosh Hashanah was also an exception in that
it was observed for two days even in Palestine, for Rosh Hashanah was also the
New Moon, the first day of Tishrei. Even in Palestine it could not always be
ascertained on the preceding day whether the particular day was the first day
of Tishrei or the last day of Elul.
In later times the astronomical calculation of the calendar became so precise
that the practice of hearing witnesses was discarded, and the Jewish calendar
was established in every detail. Despite this, the observance of the second
day of festivals, known as the "second holiday of the Diaspora," is
still retained outside of Israel.
The Jewish Festivals: A Guide to Their History and Observance
(Schocken 1996, reprinted with the permission of the Union of American Hebrew