The Tanakh contains several commandments related to the keeping of the calendar
and the lunar cycle, and records changes that have taken place to the Hebrew
For smaller units of time, see Measurement of hours below.
The Jewish day is of no fixed length. The Jewish day is modeled on the reference
to "...there was evening and there was morning..." in the Creation story.
Accordingly, it runs from sunset (start of "the evening") to the next sunset.
However, some apply special rules at very high latitudes when the sun remains
above or below the horizon for longer than a civil day.
There is no clock in the Jewish scheme, so that a civil clock is used. Though
the civil clock incorporates local adoptions of various conventions such as time
zones, standard times and daylight saving, these have no place in the Jewish
scheme. The civil clock is used only as a reference point - in expressions such
as: "Shabbat starts at ...". The steady progression of sunset around the world
and seasonal changes results in gradual civil time changes from one day to the
next based on observable astronomical phenomena (the sunset) and not on man-made
laws and conventions.
Instead of the international date line convention, there are varying opinions as
to where the day changes. One opinion uses the antimeridian of Jerusalem.
(Jerusalem is 35°13’ east of the prime meridian, so the antimeridian is at
144°47' W, passing through eastern Alaska.) Other opinions exist as well.
The Hebrew calendar follows a seven-day weekly cycle, which runs concurrently
but independently of the monthly and annual cycles. The names for the days of
the week are simply the day number within the week. In Hebrew, these names may
be abbreviated using the numerical value of the Hebrew letters, for example יום
א׳ (Day 1, or Yom Rishon (יום ראשון)):
Yom Rishon - יום ראשון (abbreviated יום א׳) = "first day" = Sunday (starting at
Yom Sheni - יום שני (abbr. יום ב׳) = "second day" = Monday
Yom Shlishi - יום שלישי (abbr. יום ג׳) = "third day" = Tuesday
Yom Reviʻi - יום רבעי (abbr. יום ד׳) = "fourth day" = Wednesday
Yom Chamishi - יום חמישי (abbr. יום ה׳) = "fifth day" = Thursday
Yom Shishi - יום ששי (abbr. יום ו׳) = "sixth day" = Friday
Yom Shabbat - יום שבת (abbr. יום ש׳) or more usually שבת - Shabbat = "Sabbath
day (Rest day)" = Saturday
The names of the days of the week are modeled on the seven days mentioned in the
Creation story. For example, Genesis 1:5 "... And there was evening and there
was morning, one day". One day also translates to first day or day one.
Similarly, see Genesis 1:8, 1:13, 1:19, 1:23, 1:31 and 2.2.
The Jewish Shabbat has a special role in the Jewish weekly cycle. There are many
special rules which relate to the Shabbat, discussed more fully in the Talmudic
In Hebrew, the word Shabbat (שַׁבָּת) can also mean "(Talmudic) week", so
that in ritual liturgy a phrase like "Yom Reviʻi bəShabbat" means "the fourth
day in the week".
Importance of lunar months
From very early times, the Mesopotamian lunisolar calendar was in wide use by
the countries of the western Asia region. The structure, which was also used by
the Israelites, was based on lunar months with the intercalation of an
additional month to bring the cycle closer to the solar cycle.
Num 10:10 stresses the importance in Israelite religious observance of the new
month (Hebrew: ראש חודש, Rosh Chodesh, "beginning of the month"): "... in your
new moons, ye shall blow with the trumpets over your burnt-offerings..."
Similarly in Num 28:11. "The beginning of the month" meant the appearance of a
new moon. In prophet Amos, the new moon seem to be described simply as
According to the Mishnah and Tosefta, in the Maccabean, Herodian, and Mishnaic
periods, new months were determined by the sighting of a new crescent, with two
eye witnesses required to testify to the Sanhedrin to having seen the new lunar
crescent at sunset. The practice in the time of Gamaliel II (c. 100 CE) was
for witnesses to select the appearance of the moon from a collection of drawings
that depicted the crescent in a variety of orientations, only a few of which
could be valid in any given month. These observations were compared against
calculations. When thirty days elapsed since the last new moon, the
witnesses were readily believed.
At first the beginning of each Jewish month was signaled to the communities of
Israel and beyond by fires lit on mountaintops, but after the Samaritans began
to light false fires, messengers were sent. The inability of the messengers
to reach communities outside Israel before mid-month High Holy Days (Succot and
Passover) led outlying communities to celebrate scriptural festivals for two
days rather than one, observing the second feast-day of the Jewish diaspora
because of uncertainty of whether the previous month ended after 29 or 30
In his work Mishneh Torah (1178), Maimonides included a chapter "Sanctification
of the New Moon", in which he discusses the calendrical rules and their
scriptural basis. He notes,
"By how much does the solar year exceed the lunar year? By approximately 11
days. Therefore, whenever this excess accumulates to about 30 days, or a little
more or less, one month is added and the particular year is made to consist of
13 months, and this is the so-called embolismic (intercalated) year. For the
year could not consist of twelve months plus so-and-so many days, since it is
said: throughout the months of the year (Num 28:14), which implies that we
should count the year by months and not by days."
Names of months
Biblical references to the pre-Jewish calendar include ten months identified by
number rather than by name. In parts of the Torah portion Noach (Noah)
(specifically, Gen 7:11, Gen 8:4-5, Gen 8:13-14) it is implied that the months
are thirty days long. There is also indication that there were twelve months
in the annual cycle (1Kin 4:7, 1Chr 27:1-15).
Many countries in the western Asian region used the Mesopotamian calendar from
very early times, though the names of months varied. Prior to the Babylonian
exile, the names of only four months are referred to in the Tanakh:
Aviv - first month - literally "spring", which originally probably meant "the
ripening of barley" (Exodus 12:2, 13:4, 23:15, 34:18, Deut. 16:1);
Ziv - second month - literally "light" (1 Kings 6:1, 6:37);
Ethanim - seventh month - literally "strong" in plural, perhaps referring to
strong rains (1 Kings 8:2); and
Bul - eighth month (1 Kings 6:38).
All of these are believed to be Canaanite names, and at least two are Phoenician
(Northern Canaanite).
During the Babylonian exile, which started in 586 BCE, Babylonian month names
were adopted, which are still in use. The Syrian calendar used in the Levant
region shares many of the names for months as the Hebrew calendar, such as
Nisan, Iyyar, Tammuz, Ab, Elul, Tishri, and Adar, indicating a common Babylonian
Hebrew names and romanized transliteration may somewhat differ, as they do for
חשוון / Marcheshvan or כסלו / Kislev: the Hebrew words shown here are those
commonly indicated e.g. in newspapers.