Introduction The Jewish calendar is lunisolar meaning that it is based on the movements of the moon and the sun. Essentially, the earth completes a full rotation on its axis in one day, the moon goes around the earth in one month (about 29 days ), and the earth circles the sun in a year (365¼ days or 12.4 lunar months). This calendar works by factoring these three elements.
Structure
The Jewish calendar consists of 12 alternating months with either 29 or 30 days each with a total of 353, 354 or 355 days in a year. An average lunar year is 354 days. It is adjusted to the solar year which has 365¼ days through the introduction of leap years to ensure that all major festivals coincide with their respective seasons.
Days
In this calendar, each day starts at sundown the day before and ends at sunset the following day. This has its origin from the Bible which states that God created light since the earth was in darkness. He called the light day, and the darkness night. Thus, every day is recorded this way with every light and darkness interval being marked as one day. Each week has seven days.
Similar to the American calendar, the first day is Sunday, and the last (Shabbat) is Saturday. There are restrictions as to which days certain holidays should fall. Such holidays include Rosh Hashana, Yom Kippur, and Sukkot. For instance, Yom Kippur should not come next to Sabbat. To avoid this, a day is either added to the month of Marcheshvan or subtracted from Kislev.
Months
The month of Nisan is the first month of the calendar while Tishri, the seventh month, is when the Jewish New Year or Rosh Hashanah is celebrated. This concept of the year beginning at a different time from the calendar year is also seen in the example of a fiscal year as used by many companies. This calendar uses lunar months that begin with the first sliver of the new moon and can either last for 29 or 30 days.
Each year has either 12 or 13 such months. A year with 12 months is shorter than a solar year by 11 days. In this scenario, holidays come 11 days earlier every successive year. If left like this, these holidays would continue appearing earlier and pass every season. To counteract this, every few years a 13th month is introduced. A 13-month year is longer than a solar year by 19 days and is called a leap year.