Since Biblical times, various astronomical phenomena have been used to establish uniquely Jewish definitions for the day and its hours, the months and the year.

The length of days and hours vary by the season, controlled by the times of sunset, nightfall, dawn and sunrise. The months and years of the Jewish calendar are established by the cycles of the moon and the sun.

Though the months follow the lunar cycle, the lunar months must always align themselves with the seasons of the year, which are governed by the sun. Thus, the Jewish calendar is "Luni-Solar." The discrepancy between the solar year (365 days) and the lunar year (354 days) was resolved by every so often adding a thirteenth month to the year, to form a "leap year."

In the early times of our history, the High Court (Sanhedrin) in Jerusalem was assigned the tasks of determining the beginning of each month and the balancing of the solar with the lunar years. They relied on direct observation of the New Moon, astronomical data, and other considerations.

In the fourth century after the Temple's destruction, however, when oppression and persecution threatened the continued existence of the Court, a fixed calendar was instituted -- based on the Sanhedrin's closely guarded secrets of calendric calculation. This is the permanent calendar according to which the New Moons and festivals are calculated and celebrated today by Jews all over the world.

Like the original system of observation, it is based on the Luni-Solar principle. It also applies certain rules by which complex astronomical calculations are combined with the religious requirements into an amazingly precise system.

The following pages will provide a brief digest of the factors which control the determination of the Jewish hour, day, month & year.

When G‑d created time, He first created night and then day. Therefore, a calendar date begins with the night beforehand. While a day in the secular calendar begins and ends at midnight, a Jewish day goes from nightfall to nightfall. Shabbat begins on Friday night, and a yahrtzeit lamp is kindled the evening before the yahrtzeit (anniversary of a person's passing), before nightfall. If the 10th of Iyar falls on a Wednesday, and a child is born Wednesday night after dark, the child's birthday is the 11th of Iyar.

On those dates wherein certain activities are restricted -- such as working on Shabbat or major holidays -- the restrictions go into effect the night beforehand.

[Most fast days begin at dawn ("alot hashachar"), and as such are an exception to this rule. Yom Kippur and Tisha b'Av, however, do begin at nightfall of the previous night.]

Though the day and its restrictions begin the night beforehand, many obligations associated with specific calendar dates -- such as hearing the shofar on Rosh Hashanah, taking the Four Species on Sukkot, or hearing the daytime reading of the Megillah on Purim -- must be preformed during daylight hours only.

Definition of Nightfall

While a day starts and ends at nightfall, the exact moment when night -- and the next calendar date -- begins is not clear.

The twilight period, from sunset ("shkiah") until three stars are visible in the sky ("tzeit hakochavim"), is an "iffy" time period, known as "bein hashmashot." Shabbat and all the holidays begin at sunset, the earliest possible definition of nightfall, and end when three stars appear in the sky the next evening, the latest definition of nightfall.

A rabbi should be consulted if a boy is born during bein hashmashot (to determine when the circumcision should be scheduled), or if a person passes away during this time (to determine the date when the yahrtzeit should be observed).

The Jewish calendar is based on lunar cycles. Towards the beginning of the moon's cycle it appears as a thin crescent. That is the signal for a new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days  – and then the thin crescent reappears, and the cycle begins again.

The entire cycle takes approximately 29.5 days. Since a month needs to consist of complete days, a month is sometimes twenty-nine days long (such a month is known as chaser, "missing"), and sometimes thirty (malei, "full").

Knowing exactly when the month begins has always been important in Jewish practice because the Torah schedules the Jewish festivals according to the days of the month.

The first day of the month as well as the thirtieth day of a malei month is called Rosh Chodesh, the "Head of the Month," and has semi-festive status.

The Jewish Months

 Nissan is the first month on the Jewish calendar. Before the Jews left Egypt, on the first day of the month of Nissan, G‑d told Moses and Aaron: "This chodesh (new moon, or month) shall be to you the head of months." Thus the peculiarity of the Jewish calendar: the year begins on Rosh Hashanah, the first day of the month of Tishrei -- the anniversary of the creation of Adam and Eve -- but Tishrei is not the first month. Rosh Hashanah is actually referred to in the Torah as "the first day of the seventh month.

Sanctifying the Month

"The L-rd spoke to Moses and to Aaron in the land of Egypt, saying, 'This chodesh shall be to you the head of months.'"

From the wording of this verse, "shall be to you," the sages extrapolated that the responsibility of pinpointing and consecrating the chodesh, the crescent new moon, was entrusted to the leaders of our nation, the Sanhedrin, the Rabbinical Supreme Court of every generation.

Originally, there was no fixed calendar. There was no way to determine in advance the exact day of a coming holiday or Bar Mitzvah, because there was no way to determine in advance when the month would begin. Each month anew, the Sanhedrin would determine whether the month would be 29 or 30 days long -- depending on when the following month's new moon was first sighted -- and would sanctify the new month.

Nowadays

In the 4th century CE, the sage Hillel II foresaw the disbandment of the Sanhedrin, and understood that we would no longer be able to follow a Sanhedrin-based calendar. So Hillel and his rabbinical court established the perpetual calendar which is followed today.

According to this calendar, every month of the year, except for three, has a set number of days:

• Nisan - 30
• Iyar - 29
• Sivan - 30
• Tamuz - 29
• Menachem Av - 30
• Elul - 29
• Tishrei - 30
• Mar Cheshvan - 29 or 30
• Kislev - 29 or 30
• Tevet - 29
• Shevat - 30
• Adar - 29 (In leap years, Adar I has 30 days)

Regarding the variable months of Kislev and Tevet, there are three options: 1) Both can be 29 days (the year is chaser), 2) both are 30 (the year is malei), or 3) Cheshvan is 29 and Kislev is 30 (the year is k'sidran, meaning these two months follow the alternating pattern of the rest of the months). Hillel also established the rules that are used to determine whether a year is chaser, malei, or k'sidran.

The rules of the perpetual calendar also ensure that the first day of Rosh Hashanah will never take place on Sunday, Wednesday or Friday.

When Hillel established the perpetual calendar he sanctified every Rosh Chodesh until Moshiach will come and reestablish the Sanhedrin.

The Sanhedrin Sanctification

The following is a brief description of the procedure the Sanhedrin followed in days of yore to determine the date of the onset of a new month.

On the 30th day of every month, the Sanhedrin would "open for business" in a large courtyard in Jerusalem called Beth Ya'azel. Witnesses who claimed to have seen the new moon on the previous night would come to give their testimony and be cross-examined.

The members of the Sanhedrin were well-schooled in astronomy. They knew exactly when the new moon would appear, and where it would be visible. Nevertheless, the sanctification of the moon depends on the crescent new moon actually being seen by two witnesses. The word "this" -- "This month shall be to you..." -- implies something that is actually seen.

The rabbis of the Sanhedrin would question the witnesses in the order of their arrival. They knew what the proper responses to their questions ought to be, and were thus quickly able to identify fraudulent claims. Starting with the elder of each pair, they would ask:"Tell us how you saw the moon –

• In which direction was it in relation to the sun?
• In which direction was its dark side in relation to the sun?
• How high in the sky did the moon appear to be?
• In which direction were the moon's tips facing?
• How wide was it?

After they had finished questioning the first witness, they would bring in his partner and question him in similar fashion. If the two accounts corroborated, the evidence was accepted.

That day, the thirtieth day, was now declared Rosh Chodesh of the new month. The head of the Sanhedrin would proclaim: "Mekudash!" ("sanctified!") and everyone would respond, "Mekudash! Mekudash!" The previous month was now retroactively determined to have had only twenty-nine days.

Publicizing the New Month

The following night (the second night of the month), huge bonfires were lit on designated mountaintops. Lookouts stationed on other mountaintops would see that a fire had been lit and would light their own fire. This chain of communication led all the way to Babylon, and even very distant communities knew that the day beforehand had been declared Rosh Chodesh.

Eventually, the Sadducees started lighting fires on the wrong days in order to manipulate the calendar. To prevent this confusion, the fire-on-mountaintop method of communication was discontinued, and instead messengers were dispatched to Babylon and all other far-flung Jewish settlements. This took a lot longer, a delay which had (and still has) halachic implications with regards to the second day of holiday observed in the Diaspora.

If no witnesses came on the thirtieth day -- either because the moon had not been "born" yet or because it was not visible -- then the next day, the thirty-first day, was automatically declared Rosh Chodesh, retroactively rendering the previous month a malei month.

Members of the Sanhedrin would go to a highly visible location where they would partake in a celebratory meal to signify the new month. No fires were lit that night. The new month is always either on the 30th or 31st day; if they hadn't lit fires the night before, it was understood that the new month started on the 31st day.

The Jewish year starts on Rosh Hashanah, "the Head of the Year," the day when Adam and Eve were created. The number of any given year (at the time of the writing of this article, the year is 5767 (2007)), is the amount of years which have elapsed since creation.

To find the corresponding Jewish year for any year on the Gregorian calendar, add 3760 to the Gregorian number, if it is before Rosh Hashanah. After Rosh Hashanah, add 3761.

Length

A standard Jewish year has twelve months; six twenty-nine-day months, and six thirty-day months, for a total of 354 days. This is because our months follow the lunar orbit, which is approximately 29.5 days. Due to variations in the Jewish calendar, however, the year could also be 353 or 355 days.

Leap Years

The Torah says, "Guard the month of the spring, and make [then] the Passover offering."Meaning, we need to ensure that Passover is celebrated in the spring.

In fact, all the biblical festivals -- Passover, Shavuot and Sukkot -- are dependant on the agriculteral seasons. Shavuot is "Chag HaBikurim" (the First Fruits Festival) and Sukkot is "Chag Ha'Asif" (the Harvest Festival). We need to make sure that all the festivals are celebrated in their proper seasons.

Thus the Jewish lunar calendar must coordinate with the cycle of the sun and the seasons which are determined by the solar orbit. The problem is that a lunar year, twelve lunar months added together, only adds up to about 354.4 days. A solar year, at almost 365.25 days, is nearly eleven days longer. If no adjustment is made, Passover would occur eleven days earlier each year, eventually drifting into winter, then fall, summer, and then spring again.

The solution is to periodically insert an extra (thirty-day) month into a year, creating a thirteen-month year. Such a year is called a shanah meuberet ("pregnant year") in Hebrew; in English we call it a leap year, and it makes up all the lunar calendar's lost days. It happens about once every three years.

The month is added to Adar, the last of the twelve months. On leap years we observe two Adars -- Adar I and Adar II.

Thus, the Jewish calendar is both lunar and solar; the months are lunar months while the years are solar years. This is in contrast to the Gregorian calendar, in which the year is a solar year and the months are formed by dividing a year into twelve parts, and is also quite different from the pure lunar calendar observed by certain religions, in which each month is determined by the moon and a year is simply twelve lunar months strung together.

  While the Sanhedrin (Rabbinical Supreme Court) presided in Jerusalem, there was no set calendar. They would evaluate every year to determine whether it should be declared a leap year.

When Hillel II instituted the perpetual calendar in anticipation of the disbandment of the Sanhedrin , he also incorporated leap years into the calendar.

Hillel's calendar runs in nineteen-year cycles, each cycle containing seven leap years: the 3rd, 6th, 8th, 11th, 14th, 17th, and the 19th years.

The length of the standard leap year is 384 days, though it could also be 383 or 385.

There are exactly fourteen different patterns that the Hebrew calendar years may take, distinguished by the length of the year and the day of the week on which Rosh Hashanah falls. Because the rules are complex, a pattern can repeat itself several times in the course of a few years, and then not recur again for a long time. But the Jewish calendar is known to be extremely accurate. It does not "lose" or "gain" time as some other calendars do.

Before the Fix

Sanhedrin considered several factors in the course of their deliberations whether to declare a leap year on a given year. The primary factor, which overrode all others, was the spring equinox. If the spring equinox would fall later than the first half of Nissan (i.e., on the 16th or later), then the year was automatically declared a leap year. This is because, as mentioned earlier, the verse states: "Guard the month of spring, and make [then] the Passover offering."

However, it wasn't enough for Passover to fall after the equinox, when it was "officially" spring; spring-like conditions needed to be evidenced. If in the land of Israel, the barley had not yet ripened, and the trees were not yet blossoming with seasonal fruit -- that, too, was sufficient reason to delay Nissan by adding a second month of Adar. Spring had to be felt; it had to be bright and green.

There were also several non-season-related factors which the Sanhedrin considered. For example, if the roads or bridges were in disrepair due to the winter rainy season, impeding the ability of the pilgrims to travel to Jerusalem for Passover.

What is Shabbat?

Shabbat is the Jewish Sabbath, celebrated every week from sundown on Friday to nightfall of Saturday. In Hebrew, Shabbat means "resting." As is recounted in the beginning of the Book of Genesis, G‑d created the world in six days and on the seventh He refrained from creating. Adam and Eve celebrated the first Shabbat in the Garden of Eden.

The observance of Shabbat by the Jewish nation is mandated in the fourth of the Ten Commandments. We sanctify the Shabbat and "rest" on this special day—defined by abstention from 39 forms of creative activity.

Practically, how is Shabbat observed and celebrated?

Briefly:

As cooking and baking is not allowed on Shabbat, much of the food preparation is done in advance.

Shabbat is ushered in on Friday afternoon the lighting of candles by women and girls. Following the special Friday night Shabbat evening prayers (preferably in the synagogue) we partake of a festive meal which is opened by the Kiddush blesingover a cup of wine or grape juice, and the Hamotzie blessing over two whole loaves of bread called Challah. The sumptuous meal is an integral part of oneg Shabbat, "delighting in the Shabbat." So we eat fish and salads, chicken or meat and other delicacies.

On Shabbat morning it's prayer time again—this service also features the weekly public Torah reading. After the prayers we again enjoy a delicious festive meal, complete with Kiddush and Hamotzie. Some time in the afternoon we enjoy a somewhat smaller meal, called Seudah Shlishit. Shabbat ends after nightfall and is marked with the brief Havdalah (separation) service which marks the departure of the holy day of rest.

On Shabbat we take a break from our mundane weekly activities. It is a time to regroup and pray, to eat and rejoice, to spend time with family and friends, to study and share—to indulge and pamper our spiritual side.

There is so much more to be said about Shabbat... Please check out the Shabbat Section which is complete with all the information you'll need.