Measuring and marking the passage of time is a universal human preoccupation. At least forty different calendars are in use around the world today.  The most widely used systems are the solar calendar, based on the cycle of the sun, and the lunar calendar, based on the cycle of the moon.  Because of its orbit around the earth, the moon alternates between being fully exposed to sunlight (full moon) and being completely shadowed by the earth (no moon – “a moonless night”).  A lunar month is defined as the time required for the moon to go from a given phase, (e.g. new moon, slim crescent) back to that phase again,  a period of about 29 ½ days.  In a lunar calendar, the month begins with the sighting of the new moon, the middle of the month coincides with the full moon and the end of the month with the gradual disappearance of the moon.  The lunar year consists of twelve lunar months, about 354 days, while the solar year is 365 ¼ days long, a difference of about eleven days.

The seasons of the year, winter, spring, summer and fall are dependent upon the earth’s rotation around the sun, its tilted orbit and its distance from the sun.  If the lunar calendar is not adjusted to the seasons of the sun at all, then a particular date will “drift,” until over the course of 33 years, it occurs in each of the four seasons.  Most of the Western world uses the Gregorian calendar which is solar.  In a solar calendar, the months are not coordinated with the cycles of the moon, and the year is based only on the seasons.  Specific dates consistently occur in the same seasons, but with no relation to the natural cycle of the moon at all (e.g. the first of the month, may coincide with a full moon, “no moon” or a quarter moon).

The Jewish calendar is a lunisolar system; it combines elements of both lunar and solar calendars.  The Jewish calendar scrupulously follows the phases of the moon, but it also incorporates features that ensure that the festivals always occur in the same seasons.  Passover, for example, always falls in the spring and Sukkos is always in autumn.  In contrast to the solar calendar, the first day of the Jewish month coincides with the first appearance of the new moon.  Complex formulae regulate the periodic adjustment of the calendar which coordinates the cycle of the moon with the cycle of the seasons. Seven times in the course of nineteen years, an extra month is added to compensate for the eleven day difference between the solar and lunar years.  A Jewish leap year, therefore, has 13 months.  The rules and astronomical measurements used to formulate the calendar are part of the Oral tradition as received on Mount Sinai and passed on from one generation to the next.

In the Torah, (as well as in modern Hebrew) the days of the week are designated by number, (i.e. “first day,” “second day”).  Only the seventh day, Shabbat, has a name.  The first chapter of Genesis speaks of  “evening and morning” when describing the creation of the world, indicating that a day is calculated from nightfall to nightfall, such that day one begins at nightfall on Saturday and ends at night on Sunday.  Originally the months were also designated only by number, beginning with the month in which the Exodus took place.  During the Babylonian exile, which followed the destruction of the First Temple,  the Jews began using Babylonian names for the months.  These names, Nissan, for the first month, Iyar,  for the second month, etc., are still in use today.

In the times of the Temple (from about 832 BCE until 70 CE), the highest Jewish court in Israel, the Sanhedrin, would determine when a new month began.  The beginning of each month was (and still is) marked by a semi-holiday known as Rosh Chodesh.  Eye-witness reports that a new moon had been sighted were critical in declaring a Rosh Chodesh, but astronomical calculations, agricultural, economic and religious considerations were also taken into account  The process of determining the beginning of the new month is called Kiddush Hachodesh, the Sanctification of the New Moon.  Setting Rosh Chodesh ahead or back a day determines when the upcoming festivals will begin and therefore the holiness of the festival days is thus created by Kiddush Hachodesh.

Every member of the Sanhedrin had authority conferred upon him by someone else who had authority conferred upon him, in an unbroken chain all the way back to Moses who gave authority to Joshua.  During the period of persecution by the Roman Empire, the Sanhedrin lost much of its power.  Early Roman decrees made it illegal to grant or to receive this authority,   and when the Roman empire became Christian, under Constantine, they forbade the Sanhedrin even to convene.  Since Rosh Chodesh can only be declared by the meeting of a Sanhedrin whose members have semichah, it became increasingly difficult and dangerous to sanctify the new moon each month.  By about 360 CE the Rabbis realized that the situation was only getting worse and the time would come when a Sanhedrin with semichah would no longer exist.  Hillel II and his court therefore decided at this time to establish a fixed calendar and to calculate, sanctify and declare all subsequent months and years.  The calendar that we use today is based on these calculations and sanctifications.

In the times of the Temple, the Sanhedrin would decide when to sanctify a new month primarily on the basis of eyewitness reports.  Two people would come to the court and testify that they had seen the new moon.  If the court decided to accept the testimony, they would immediately declare that day Rosh Chodesh, making the previous month 29 days long.   The Sanhedrin could delay the declaration of the new month until the next day for economic or social considerations, or if they rejected the testimony because it was ambiguous or incorrect.  The previous month would then be 30 days long since a lunar month must be either 29 or 30 days.  The declaration of Rosh Chodesh would determine when the festivals would begin, “moving” them either ahead or back by a day.  It was therefore necessary for the Sanhedrin to inform the dispersed Jewish communities of the declaration of the new month as soon as possible, so that people could observe the festivals at the correct times.  The court would send messengers out to inform as many people as possible of the new month before the onset of the next festival.  The communities outside of Israel, however, were often too far away for the messengers to reach in time, and would remain in doubt about the day on which the festival would begin.  These communities, known as the Diaspora, therefore observed the observed the “holy days” of each festival for two days instead of one to account for the two possible days on which Rosh Chodesh may have occurred.   The only exception to this is Rosh Hashanah, which is the only festival that begins on the first of the month.  Therefore, it was (and is) always observed for two days even in Israel since the messengers were clearly unable to tell people in time when Rosh Chodesh had been declared.  Diaspora communities observe the Day of Atonement, Yom Kippur, for only one day, since it is a twenty five hour fast, and there is a health risk involved in fasting for two whole days.

Once Hillel II established the set calendar there was no longer any doubt about the day on which any given festival was to be celebrated.  Nevertheless, the Sages decreed that the Diaspora custom of keeping two-day festivals should remain in force and commanded the Jews of the Diaspora to “keep the custom of their fathers.”  The Talmud explains that this was done in order to guard against the possibility that the calendar system might collapse in a time of persecution.  Should this occur we would once again be in doubt about the when to observe the festival.  Some commentaries explain that because of the intensity of the spirituality in the Land of Israel, one day is sufficient to absorb the lessons of the festivals, while outside of Israel two are required.  Others suggest that we retain this custom as a way of experiencing the idea expressed in the verse in Isaiah,  “For from Zion shall come forth the law and the word of God from Jerusalem.”   Even though we have a fixed calendar today, we remember and hope for a return to the time when we looked to the Sanhedrin in Jerusalem to hear “the word of God.”

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