Three independent phenomena are the basis for the Jewish calendar; rotations of the earth about its axis, cycles of the moon around the earth and orbits of the earth around the sun. These phenomena are responsible for days, months and years, respectively. This basis is different to the basis of the civil calendar which doesn't correlate between moon cycles and the month.
The Jewish lunar months begin when the first fragment of moon is visible after the Dark Moon (i.e. when the moon isn't visible). There are roughly 12.4 lunar months per solar year so the lunar years have eleven less days than a solar year. Therefore, an extra month is occasionally added to the Jewish calendar. This is to make sure that the festival of Passover, which is also called the Festival of Spring actually occurs in the Spring!
When an extra month is added, it is at the end of the year. In such cases there are two months of the Hebrew month of Adar instead of one, with the festival of Purim being celebrated in the second month.
Therefore, some years will have twelve months and others thirteen!
The years in the Jewish calendar are according to the number of years that have transpired since creation. The years 2010-2011 are parallel to the Hebrew year of 5771.
- Adar II (in the event of a leap year)
Although Nissan is considered the first month of the Jewish year, the Jewish New Year, Rosh Hashanah, falls out in Tishrei. Therefore a year is added each time Tishrei comes around.
The day of rest, Shabbat, is the only day with a name. All the other days are simply numbered. Sunday is known as the first day, Monday the second day etc.
By the way...
- The Jewish year always straddles two years in the Gregorian calendar. This means that spring holidays occur in the civil year following the civil year in which Rosh Hashanah took place. For example, Rosh Hashana in the Hebrew year of 5758 took place in September 1997 and Passover in the same Hebrew year took place in April 1998.
- On some Jewish holidays Jews in the Diaspora will celebrate two days whereas Jews in Israel will celebrate only one. This is due to the difference in time zones between the countries and ensures that the Jews in the Diaspora will celebrate the holiday throughout the entire time that it is celebrated in Israel.