Months of the Jewish Year
The months of the Jewish year are lunar in nature.
Unlike the months of the Gregorian solar year that is the norm in the world today, the months of the Jewish year reflect the phases of the moon. This can be seen most clearly in the length of the months. Whereas the months of the Gregorian calendar vary in length between twenty-eight and thirty-one days in order to make a solar year of 365 (or, in leap years, 366) days, the months of the Jewish year are either twenty-nine or thirty days long. This reflects the fact that a lunar month is twenty-nine and a half days in length, and the months always must begin with the new moon.
A year of twelve lunar months, however, is some eleven days shorter than a solar year. In order to ensure that the various seasonally based holidays in the Jewish calendar continue to occur at the correct season, the rabbis developed a system over time that allowed them to coordinate their lunar months with the solar year by inserting a leap month at the end of the year seven times in every 19-year cycle. This is now fixed in the third, sixth, eighth, 11th, 14th, 17th, and 19th years of the cycle. Although this is traditionally ascribed to Rabbi Hillel II in the fourth century CE, it is probable that the system in use today developed slowly during the course of the mid to late first millennium.
In order to further fine-tune their calculations, the rabbis determined that the months of Nisan (March-April), Sivan (May-June), Av (July-August), Tishrei (September-October), and Shevat (January-February) are always thirty days long. Iyyar (April-May), Tammuz (June-July), Elul (August-September), Tevet (December-January), and Adar (March-April) are always twenty-nine days long. Heshvan (October-November) and Kislev (November-December) are either twenty-nine or thirty days in length. In a leap year, there are two months of Adar, the last month of the year. When that occurs, Adar I is thirty days long, and Adar II twenty-nine. A short Jewish year, therefore, consists of 353 to 355 days, while a leap year varies between 383 and 385 days.
The names that we use for the Jewish months are actually Babylonian in origin and were adopted by the Jews as of the time of the Babylonian exile in the sixth century BCE. The Bible indicates that until then the months were oftentimes called simply by their numerical position in the year (First Month, Second Month, etc.), just as the days of the week--with the exception of Shabbat--still are in Hebrew. In addition, the Bible does record some ancient names for the months that disappeared once the Jews adopted the Babylonian names. These include the now forgotten months of Bul and Aviv, among others. The Gezer Calendar from the 10th century BCE, arguably the oldest Hebrew inscription ever discovered, refers to the months according to the agricultural activities associated with them.
The Jewish month begins with the first sighting of the new moon, the Rosh Chodesh. There are special prayers associated with the beginning of the month, and Rosh Chodesh ceremonies have oftentimes played an important role particularly among the female members of the Jewish community.
Although the Jewish new year is celebrated at the beginning of Tishrei (Rosh Hashanah), this month is actually the seventh month according to ancient reckoning. The first month is actually Nisan, during which Passover (Pesach) falls. In this manner, the Jewish year begins with God’s great redemptive act at the time of the Exodus from Egypt.
Holidays and festivals are scattered throughout the Jewish year, with the exception of the month of Heshvan. Therefore, this month has also been termed Marheshvan, bitter Heshvan, since it lacks a holiday. However, the term "mar" could also be read as "mister," which is also interpreted midrashically to mean that this poor month without a holiday is compensated by receiving special respect!