The current Jewish calendar is generally said to have been set down by the Sanhedrin president Hillel II in approximately CE 359. The original details of his calendar are, however, uncertain. The Jewish calendar is used for religious purposes by Jews all over the world, and it is the official calendar of Israel.

Holidays are celebrated on the same day of the Jewish calendar every year, but the Jewish year is not the same length as a solar year or the Gregorian calendar used by most of the western world, so the dates shift on the Gregorian calendar. The dates of Jewish holidays do not change from year to year.

The Jewish calendar is primarily lunar, with each month beginning on the new moon, 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 difficulty 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 calendar gains about 19 days every year. The months on such a calendar 'drift' relative to the solar year. On a 12-month calendar, the month of Nissan, which is supposed to occur in the Spring, occurs 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: a second month of Adar. 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 the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. 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. Adar II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The current cycle began in the Jewish year 5758 (the year that began October 2, 1997).

In addition, Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabba 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 occurrences from happening.

Calculating the New Moon

In order to understand the calculations, one must know that an hour is subdivided into 1080 'parts'. The new moon that started the year AM 1, occurred 5 hours and 204 parts after sunset (that is, just before midnight on Julian date 6 October 3761 BCE). The new moon of any particular year is calculated by extrapolating from this time, using a synodic month of 29 days 12 hours and 793 parts.

Numbering of Jewish Years

Many Orthodox Jews will readily acknowledge that the first six "days" of creation are not necessarily 24-hour days (indeed, a 24-hour day would be meaningless until the creation of the sun on the fourth "day"). A Jewish-calendar day does not begin at midnight, but at either sunset or when three medium-sized stars should be visible, depending on the religious circumstance.

The year number on the Jewish calendar represents the number of years since creation, calculated by adding up the ages of people in the Torah (mostly the Old Testament of the Christian Bible) back to the time of creation. That was on 6 October 3761 BCE. Now, according to Jewish tradition, 1,000 years count as one day with God. This means that when the seventh millennium begins for the Jewish calendar, the seventh day begins for God. According to Jewish concepts, this is when the Mashiah will come again.

Jews do not generally use the words AD and BC to refer to the years on the Gregorian calendar. A.D. means Anno Domini, and it was used before dates after the supposed year Christ was born. Jewish people do not believe in Jesus as Christians do. Instead, they use the abbreviations CE (Common or Christian Era) and BCE (Before the Common Era).

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 most of the Bible refers to months by number, not by name.

The Jewish calendar has the following months:

Hebrew	English	Number	Length	Gregorian Equivalent
	Nissan	1	30 days	March - April
	Iyar	2	29 days	April - May
	Sivan	3	30 days	May - June
	Tammuz	4	29 days	June - July
	Av	5	30 days	July - August
	Elul	6	29 days	August - September
	Tishri	7	30 days	September - October
	Cheshvan	8	29 or 30 days	October - November
	Kislev	9	30 or 29 days	November - December
	Tevet	10	29 days	December - January
	Shevat	11	30 days	January - February
	Adar	12	29 or 30 days	February - March
	Adar II	13	29 days	March - April

In leap years, Adar has 30 days. In non-leap years, Adar has 29 days.

The length of Cheshvan and Kislev are determined by complex calculations involving the time of day of the full moon and the day of the week that Tishri would occur in the following year. If you want to play with these complex calculations there are plenty of easily accessible computer programs that will calculate out the Jewish calendar for more than a millennium.

Note that the number of days between Nissan and Tishri is always the same. Because of this, the time from the first major festival (Passover in Nissan) to the last major festival (Sukkot in Tishri) is always the same.

Years are counted since the creation of the world, which is assumed to have taken place in 3761 BCE In that year, AM 1 started (AM = Anno Mundi = year of the world).

In the year CE 2004 we have witnessed the start of Jewish year AM 5765.

Please NOTE that the above information is just a brief explanation of the Jewish calendar. More information can be found at libraries.

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