The Jews follow the lunar calendar. It is reckoned from 3761 BC, the year of the creation of the world according to Jewish belief. It has 12 months of 29 or 30 days each. Therefore, one year has 354-355 days.
The seven days of the week are Yom Echad (Sunday), Yom Sheni(Monday), Yom Shelishi (Tuesday), Yom Raveyi (Wednesday), Yom Khameshi (Thursday), Yom Sheshi(Friday) and Sabbath (Saturday). Sabbath is the day of rest. According to traditional Jewish belief, God created the world in six days, and on the seventh day (Sabbath) he rested. It is therefore considered the rest day. On this day, there are additional morning and afternoon prayers, besides the three daily prayers.
To keep pace with the Gregorian calendar, an extra month of 29 days called Ve-Adar is added every 3 years. This month falls between the months of Adar and Nisan.
The Jewish year begins on the first day of Tishri, when the new moon appears nearest to the autum nequinox. This date shifts from time to time according to ancient regulations which specify that New Year`s Day must not fall on a Friday or a Sunday. If it did, it would mean two consecutive days on which certain tasks may not be performed. This would result in practical problems; for instance, no Jew can be left unburied for two days.
According to the Jewish calendar, a day consists of the time from sundown until the next sundown. As a result, all Jewish holidays begin at sundown.