Hebrew calendar

[9] The Hebrew week (שבוע, shavua) is a cycle of seven days, mirroring the seven-day period of the Book of Genesis in which the world is created.

Although originally the new lunar crescent had to be observed and certified by witnesses (as is still done in Karaite Judaism and Islam), nowadays Jewish months have generally fixed lengths which approximate the period between new moons.

The insertion of the leap month is based on the requirement that Passover occur at the same time of year as the spring barley harvest (aviv).

From the eleventh century, anno mundi dating became the dominant method of counting years throughout most of the world's Jewish communities, replacing earlier systems such as the Seleucid era.

The reference junction of the Sun and the Moon (Molad 1) is considered to be at 5 hours and 204 halakim, or 11:11:20 p.m., on the evening of Sunday, 6 October 3761 BCE.

In English, the code consists of the following: The kevi'ah in Hebrew letters is written right-to-left, so their days of the week are reversed, the right number for 1 Tishrei and the left for 15 Nisan.

Its remarkable accuracy (less than one second from the current true value) is thought to have been achieved using records of lunar eclipses from the 8th to 5th centuries BCE.

The rate of calendar drift is increasing with time, since the mean synodic month is progressively shortening due to gravitational tidal effects.

[p] When the calendar was fixed in the 4th century, the earliest Passover (in year 16 of the Metonic cycle) began on the first full moon after the March equinox.

Calendar drift also impacts the observance of Sukkot, which will shift into Israel's winter rainy season, making dwelling in the sukkah less practical.

Modern scholars have debated at which point the drift could become ritually problematic, and proposed adjustments to the fixed calendar to keep Passover in its proper season.

[57] The seriousness of the calendar drift is discounted by many, on the grounds that Passover will remain in the spring season for many millennia, and the Torah is generally not interpreted as having specified tight calendrical limits.

However, some writers and researchers have proposed "corrected" calendars (with modifications to the leap year cycle, molad interval, or both) which would compensate for these issues: Religious questions abound about how such a system might be implemented and administered throughout the diverse aspects of the world Jewish community.

[61] Canaan State of Israel (1948–present) Early Zionist pioneers were impressed by the fact that the calendar preserved by Jews over many centuries in far-flung diasporas, as a matter of religious ritual, was geared to the climate of their original country: major Jewish holidays such as Sukkot, Passover, and Shavuot correspond to major points of the country's agricultural year such as planting and harvest.

The Hebrew calendar is still widely acknowledged, appearing in public venues such as banks (where it is legal for use on cheques and other documents),[62][63] and on the mastheads of newspapers.

[67] According to some Christian and Karaite sources, the tradition in ancient Israel was that 1 Nisan would not start until the barley is ripe, being the test for the onset of spring.

[95] The Talmud states: Rav Aha bar Jacob then put this question: How do we know that our Era [of Documents] is connected with the Kingdom of Greece at all?

[96]In the 8th and 9th centuries, as the center of Jewish life moved from Babylonia to Europe, counting using the Seleucid era "became meaningless", and thus was replaced by the anno mundi system.

[98] When the observational form of the calendar was in use, whether or not a leap month was added depended on three factors: 'aviv [i.e., the ripeness of barley], fruits of trees, and the equinox.

[102] 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.

Elsewhere, Shimon ben Pazi is reported to have counseled "those who make the computations" not to set Rosh Hashana or Hoshana Rabbah on Shabbat.

Later writers, such as Nachmanides, explained Hai Gaon's words to mean that the entire computed calendar was due to Hillel II in response to persecution of Jews.

Maimonides (12th century) stated that the Mishnaic calendar was used "until the days of Abaye and Rava" (c. 320–350 CE), and that the change came when "the land of Israel was destroyed, and no permanent court was left."

H. Graetz[117] linked the introduction of the computed calendar to a sharp repression following a failed Jewish insurrection that occurred during the rule of the Christian emperor Constantius and Gallus.

Saul Lieberman argued instead that the introduction of the fixed calendar was due to measures taken by Christian Roman authorities to prevent the Jewish patriarch from sending calendrical messengers.

In the lunar Karaite calendar, the beginning of each month, the Rosh Chodesh, can be calculated, but is confirmed by the observation in Israel of the first sightings of the new moon.

The addition of the leap month (Adar II) is determined by observing in Israel the ripening of barley at a specific stage (defined by Karaite tradition) (called aviv),[130] rather than using the calculated and fixed calendar of rabbinic Judaism.

[134][135] The Sardica paschal table shows that the Jewish community of some eastern city, possibly Antioch, used a calendrical scheme that kept Nisan 14 within the limits of the Julian month of March.

[136] Some of the dates in the document are clearly corrupt, but they can be emended to make the sixteen years in the table consistent with a regular intercalation scheme.

The inscriptions, however, reveal no clear pattern of regular intercalations, nor do they indicate any consistent rule for determining the start of the lunar month.