The Jewish calendar reflects the cycles of the sun and moon, as well as astronomical phenomena and the seasonal changes in the Land of Israel. It guides us through the cycles of the year, and also aids our recollection of significant historical moments in our history. So, for example, during Pesach we may watch the wheat ripen and also recall that God redeemed us from slavery in this season.
Early Zionist pioneers noted that the religious calendar reflected the climate of our original homeland and used this calendar for agriculture. Today it is both the civil and religious calendar in Israel.
Days
The Jewish day begins at sunset per the account of creation in Breisheet. “There was setting, there was dawning, day one.”
The length of the days and the hours vary by season. Days are longer in summer and shorter in the winter, so the hours of each day vary accordingly. An hour is one-twelfth of the time between sunrise and sunset.
A new day starts after sunset, when three stars are visible in the sky. (This would have occurred earlier in ancient time, when there was little light pollution obscuring our view of the heavens.) Twilight, the liminal time between the moment the sun disappears below the horizon and the moment three stars are visible in the sky, is called, בֵּין הַשְׁמָשׁוֹת, meaning ‘between the two suns.’
Weeks
The seven-day cycle of weeks continues regardless of months, seasons, or years, and every seventh day is a Sabbath of rest, during which we emulate God by refraining from creative work.
Every seventh year is considered a Sabbatical year, and today, some farms in Israel observe both Sabbatical and Jubilee years.
Months
Months begin when the first sliver of the new moon is visible around sunset. The moon waxes for about two weeks until it is full. After it is full, the moon wanes for about two more weeks until it becomes invisible for approximately two days. Then the next new lunar crescent appears.
The synodic month is 29½ days. So, to ensure the calendar remains in sync with the actual phase of the moon, a calendar month is sometimes twenty-nine days long and sometimes thirty. The first day of the month and the thirtieth day of a malei (full) month, are called Rosh Chodesh, the “Head of the Month,” and are a semi-festival.
There are twelve lunar months, alternating between 29 and 30 days. (The new moon of Tishrei is significant because it is the primary determiner of the date of Rosh HaShanah.) The first month, Nisan, has 30 days and the next month, Iyyar, has 29. The number of days alternates between 30 and 29 each month throughout the year, until Cheshvan or Kislev. Following those two months, Tevet always has 29, Shevat 30, and Adar 29.
Nisan 30 days
Iyyar 29 days
Sivan 30 days
Tammuz 29 days
Av 30 days
Elul 29 days
Tishrei 30 days
Cheshvan 29 or 30
Kislev 29 or 30
Tevet 29 days
Shevat 30 days
(Adar Alef 30 days)
Adar/AdarBet 29 days
An extra day may be added to Cheshvan or removed from Kislev to prevent Yom Kippur from falling on a Friday or Sunday or to prevent Hoshannah Rabbah from falling on Shabbat. Both months can be 29 days, both can be 30 days, but if Cheshvan has 29 days, then Kislev must have 30. (Yom Kippur cannot fall on a Friday or Sunday, or there would be two consecutive days when preparing food and burying the dead are prohibited; Hoshana Rabbah cannot occur on Shabbat, or we could not carry a lulav on this day.)
In ancient times, the Sanhedrin would declare a new month after two eyewitnesses had seen the lunar crescent at sunset. Although members of the Sanhedrin were well versed in astronomy and had mathematical models to correctly predict when the new moon would appear, a verse in Shemot was understood to mean people were responsible for witnessing and consecrating the new moon. Witnesses would appear in pairs and be questioned individually. If their testimony was accepted, the new month was declared. Specific questions were asked to verify their testimony.
Many Jewish festivals begin during the full moon, the fifteenth day of the month. The new moon, Rosh Chodesh, is a holiday for women. Oral tradition says that when the Israelites who had just left Egypt created a golden calf to worship during Moses’s absence, the Israelite women refused to participate in its creation or worship. In ancient times, women rested from work during the new moon.
Leap Month
A year of 12 lunar months is shorter than a solar year; it is 354 days rather than 365. To ensure that Pesach is observed in the springtime near the equinox, a leap month, is added in seven times every 19 years. (Every 19 years, the phases of the moon recur on the same day of the year.)
In ancient times, Adar Bet was added to a year based on astronomical events and observation of the ripening barley and fruit on trees. If spring had not arrived, a second Adar was added. By today’s mathematical calculations, Adar Alef is considered the extra month.
Seasons
The autumn and winter months (Elul through Shevat) are called the days of rain. The spring and summer months (Adar through Av) are called the days of sun. The three pilgrimage festivals of Pesach, Shavuot, and Sukkot occur during spring, summer, and autumn.
Years
The procedure for declaring the new month was different for the new moon of Tishrei, which is also the beginning of the New Year. Rosh Hashannah begins on the first day of the seventh month. We couldn’t wait for witnesses to sanctify Rosh Chodesh Tishrei. So on the evening following 29 Elul, the court would sanctify the day as the first of Tishrei. If witnesses appeared the next day, the day could be retroactively established as Rosh HaShanah on that day. If witnesses did not appear, then the following day would be Rosh Hashanah and retroactively, the previous evening would be a regular weekday. To prevent people from treating the first day lightly, it was decided that Rosh Hashanah would be celebrated as a two-day holiday.
There are four new years in the Hebrew calendar:
• Nisan – for measuring a king’s reign and marking the beginning of pilgrimages
• Elul – for the tithing of animals
• Tishrei – the High Holy Days
• Tu B’Shevat – measuring the age of trees
Year/Epoch
Shortly before the Sanhedrin ceased functioning, Hillel HaNasi established a perpetual calendar based on mathematical calculations. In the Middle Ages, Maimonides codified these calculations. He also changed the naming of the epoch. No longer do we date events “since the Destruction of the Temple.” Now the epoch is “since the Creation of the world.” (Anno Mundi 1 is the year before creation. As I write this, it is currently A.M. 5781.)
