Essentially, it is a calendar arranged according to the orbit of the Moon. The calendar was created by reconciling the lunar orbit periods with the solar Gregorian calendar, which is arranged according to the orbit of the Sun, in order to bring both religious, agricultural and astrological periods to the same season every year. It is a compositional calendar cycle consisting of 19 years where 7 years contain 13 months and 383-385 days. The Hebrew Calendar year begins on the first day of the month of Tishrei and ends on the 29th day of the month of Elul.
Since the holidays and mourning days in Judaism are calculated according to the Hebrew Calendar, the holidays do not always fall on the same date, but they always fall in the same season.
In Autumn, there is the Jewish New Year (Rosh Hashanah), followed ten days later by a 26-hour fast called Yom Kippur, which means the Day of Atonement. Five days after Yom Kippur, there is Sukkot (the Feast of Tabernacles) which lasts 7 days, and at the end of Sukkot there is Simchat Torah (the completion of the reading of the whole Torah within a year). Hanukkah is in December, while Purim commemorating the the last moment prevention of the massacre of the Jews living in Persia is around the end of February and March. In April there is Pesach, which celebrates the exodus from Egypt and liberation from slavery under the leadership of Moses (Moshe Rabenu). Shavuot which came 49 days later after Pesach, celebrates the receiving of the 10 Commandments. In Summer, there is the fast of Tisha Beav which is a day of mourning to commemorate the destruction of the Temple along with various other unfortunate events in the Jewish history.
Rosh Hashanah and Yom Kippur are named Yamim Noraim (High Holiday sor Days of Awe) while Pesach, Shavuot and Sukot are called Shalosh Regalim (Three Pilgrimage Festivals)
