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The lunar year comprises (in a normal year) twelve months each of 29 or 30 days. In a leap year a thirteenth month is added, known as Adar II. A leap year occurs seven times in each cycle of nineteen years; in the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteenth years. By adding the extra month, the lunar year of 354 days is made to harmonise with the solar year of 365 days.
The Hebrew names of the months were adopted from the Babylonian calendar during the Babylonian exile in 586 B.C.E. The first written calendar was compiled by Hillel II in 359 C.E. The days of the New Moon are considered important days in the Jewish calendar and are known as Rosh Chodesh.
