The Jewish calendar and Jewish holidays are guided by the variation in the manner in which the days are reckoned. In most other parts of the world, a day is reckoned as the period from midnight to midnight. But, for the Jews, the day denotes the period from sunset to sunset. For this reason, Jewish holidays will commence at the sunset of the day prior to the holiday mentioned in the Jewish calendar. Similarly, the holiday would end with the sunset on the date mentioned in Jewish calendar for the particular Jewish holidays.
The Jewish Holidays
To understand the precise timing of Jewish holidays, you should consult a Jewish calendar or the Chabad or Orthodox Union closest to you. No work is permitted on some of the Jewish holidays and these include Rosh Hashanah, Shemini Atzeret, Simchat Torah Shavu'ot and Yom Kippur. Similarly, the first two days of Sukkot, the first two days, - the seventh and eight day, of 'Passover' are also prohibited days for work.
The nature of work prohibited follows the same as 'Shabbat' with the exception of baking, cooking, carrying and transferring fire (though these too are prohibited on 'Shabbat'). However, if Jewish holidays fall on the 'Shabbat' day, all restrictions applicable on a 'Shabbat' day will be followed. For the devout Jews working in the secular world, this can at times pose a problem since he may have to take 13 days off from work to be able to observe these Jewish holidays.
An extra day to celebrate
Some Jewish holidays also have an extra day of celebrations. This practice came into being several hundred years ago, when personal messengers were the only form of communication. Nevertheless, these extra days are not celebrated by Israelis even when they are away from Israel. Similarly, those Jews visiting Israel will celebrate the extra Jewish holidays.
The Jewish Calendar
The Jewish calendar is governed by three astronomical aspects. The earth's rotation on its axis comprising a day, the moon's revolution around the earth denoting a month and the Earth's revolution around the sun denoting an year make up the three aspects. These three factors being independent of each other, no correlation can be established between them. Revolution of the moon around the earth takes about 29.5 days on an average. The earth revolves around the sun in 365¼ days constituting about 12.4 lunar months. The Jewish calendar coordinates the 3 astronomical phenomenons. The months in the Jewish calendar are therefore either 30 or 29 days and the years having 12 or 13 months to account for the 12.4 months of the solar revolution.
The Extended Year
Certain years in the Jewish calendar will have 13 months. This occurs because lunar calendars have about 12.4 months in a year. A strict 12 month lunar calendar will be shorter by 11 days compared to the solar year and similarly the 13 month year will be longer by 19 days in relation to the solar year. This type of extended year in a Jewish calendar is known in Hebrew as Shanah Me'uberet meaning 'pregnant' the English equivalent of a leap year.