The Jewish Calendar:

Background and History: 

The Jewish calendar is based on three astronomical phenomena: the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year). These three phenomena are independent of each other, so there is no direct correlation between them. On average, the moon revolves around the Earth in about 29½ days. The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.

The Gregorian calendar used by most of the world has abandoned any correlation between the moon cycles and the month, arbitrarily setting the length of months to 28, 30 or 31 days.

The Jewish calendar, however, coordinates all three of these astronomical phenomena. Months are either 29 or 30 days, corresponding to the 29½-day lunar cycle. Years are either 12 or 13 months, corresponding to the 12.4 month solar cycle.

The lunar month on the Jewish calendar begins when the first sliver of moon becomes visible after the dark of the moon. In ancient times, the new months used to be determined by observation. When people observed the new moon, they would notify the Sanhedrin. When the Sanhedrin heard testimony from two independent, reliable eyewitnesses that the new moon occurred on a certain date, they would declare the rosh chodesh (first of the month) and send out messengers to tell people when the month began.

The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar loses about 11 days every year and a 13-month lunar gains about 19 days every year. The months on such a calendar "drift" relative to the solar year. On a 12 lunar month calendar, the month of Nissan, which is supposed to occur in the Spring, would occur 11 days earlier each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again. To compensate for this drift, an extra month was occasionally added. The month of Nissan would occur 11 days earlier for two or three years, and then would jump forward 29 or 30 days, balancing out the drift. In ancient times, this month was also added by observation: the Sanhedrin observed the conditions of the weather, the crops and the livestock, and if these were not sufficiently advanced to be considered "spring," then the Sanhedrin inserted an additional month into the calendar to make sure that Passover, the festival of Spring, would occur in the spring.

A Jewish leap year has 13 months. We add an additional month to the year. In the Torah's calculation, the first month is Nissan, and the last month is Adar. Since Adar is the last month of the year, we add a second Adar and make the year "pregnant (שנה מעוברת)."

Establishment of our current "fixed" Jewish Calendar:

In the fourth century, Hillel II foresaw the dispersal of Jews and the eventual demise of the Sanhedrin as an institution. If there would be no Sanhedrin, there could be no Jewish calendar! So the Sanhedrin, in Hillel's time, established a fixed calendar based on mathematical and astronomical calculations. When they had that calendar, they sanctified all of the beginnings of each month of that calendar until the coming of the Messiah when once again the months would be sanctified through the testimony of witnesses regarding the new moon.

This calendar, still in use, standardized the length of months and the addition of months over the course of a 19-year cycle, so that the lunar calendar realigns with the solar years. The extra Adar is added in the 3^rd, 6^th, 8^th, 11^th, 14^th, 17^th and 19^th years of the cycle.

In addition, Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah should not fall on Saturday because it would interfere with the holiday's observances. A day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening. This process is sometimes referred to as "fixing" Rosh Hashanah.

Numbering of Jewish Years

The year number on the Jewish calendar represents the number of years since creation, calculated by adding up the ages of people in the Bible back to the time of creation.

Jews do not generally use the abbreviations "A.D." and "B.C." to refer to the years on the Gregorian calendar.  Instead, we use the abbreviations C.E. (Common Era) and B.C.E. (Before the Common Era), which are commonly used by scholars today.

Months of the Jewish Year

The "first month" of the Jewish calendar is the month of Nissan, in the spring, when Passover occurs. However, the Jewish New Year is in Tishri, the seventh month, and that is when the year number is increased. This concept of different starting points for a year is not as strange as it might seem at first glance. The American "new year" starts in January, but the new "school year" starts in September, and many businesses have "fiscal years" that start at various times of the year. Similarly, the Jewish calendar has different starting points for different purposes.

The names of the months of the Jewish calendar were adopted during the time of Ezra, after the return from the Babylonian exile. The names are actually Babylonian month names, brought back to Israel by the returning exiles. Note that of the Bible refers to months by number, not by name.

The Jewish calendar has the following months:

In a regular year:

In a leap year: