The Jewish religious year with its feasts and fasts. Whatever its origins (the question of which is extremely complicated) the Jewish calendar, from at least the beginning of the present era, was a lunar calendar. The very word for ‘month’, hodesh, means ‘that which is renewed’ and refers to the waning and waxing of the moon; hence the biblical name Rosh Hodesh, the ‘head of the month’, for the beginning of the month, observed as a festival in biblical times (I Samuel 20: 24).
The problem at the heart of the Jewish calendar is that while it is lunar, there is a need to bring it into relationship with the solar year. This is because the months are to be counted from the month in which the Exodus from Egypt took place (Exodus 12: 2) and yet the festival of Passover, celebrating the Exodus, is said (Deuteronomy 16: 1) to fall in the month Aviv (‘ripening’ of the corn); Passover must fall in spring. Since the lunar year is shorter than the solar, something had to be done to prevent Passover moving through the solar year so as to fall in other than the spring month. The method adopted was to intercalate the lunar year, that is, to add an extra lunar month to seven years in a cycle of nineteen lunar years. (The leap years, the years which have an additional month, are the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth of the nineteen year cycle, the beginning of the cycle being established by tradition.)
The names of the months of Babylonian origin, as we now have them, are found only in late books of the Bible such as the book of Esther. These names, used universally by Jews, are, counting from the first month, the month in which Passover falls: Nisan, Iyyar, Sivan, Tammuz, Av, Elul, Tishri, Marheshavan, Kislev, Tevet, Shevat, Adar. In a leap year the added month is always the one before Nisan and is called Adar Sheni, ‘Second Adar’. See The Jewish Calendar.