Passover | Jewish holy day and festival

Passover is a Jewish holy day and festival. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt. Passover begins on the 15th day of the month of Nisan, which is spring in the Northern Hemisphere, and is celebrated for seven or eight days. It is one of the most widely observed Jewish holidays.

Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar. In accordance with the Hebrew Bible, Nisan is the first month of the Hebrew calendar's festival year. Passover is a spring festival, so the 14th day of Nisan begins on the night of a full moon after the vernal equinox. To ensure that Passover did not start before spring, the tradition in ancient Israel held that the first day of Nisan would not start until the barley is ripe, being the test for the onset of spring. If the barley was not ripe, or various other phenomena indicated that spring was not yet imminent, an intercalary month (Adar II) would be added. However, since at least the 4th century, the date has been fixed mathematically.

In Israel, Passover is the seven-day holiday of the Feast of Unleavened Bread, with the first and last days observed as legal holidays and as holy days involving abstention from work, special prayer services, and holiday meals; the intervening days are known as Chol HaMoed ("festival days"). Diaspora Jews historically observed the festival for eight days, and most still do. Reform and Reconstructionist Jews and Israeli Jews, wherever they are, usually observe the holiday over seven days. The reason for this extra day is due to enactment of the Sages. It is thought by many scholars that Jews outside of Israel could not be certain if their local calendars fully conformed to practice of the Temple at Jerusalem, so they added an extra day. But as this practice only attaches to certain (major) holy days, others posit the extra day may have been added to accommodate people who had to travel long distances to participate in communal worship and ritual practices; or the practice may have evolved as a compromise between conflicting interpretations of Jewish Law regarding the calendar; or it may have evolved as a safety measure in areas where Jews were commonly in danger, so that their enemies would not be certain on which day to attack. 

Karaite Jews and Samaritans use different versions of the Jewish calendar, which are often out of sync with the modern Jewish calendar by one or two days. In 2009, for example, Nisan 15 on the Jewish calendar used by Rabbinical Judaism corresponds to April 9. On the calendars used by Karaites and Samaritans, Abib or Aviv 15 (as opposed to 'Nisan') corresponds to April 11 in 2009. The Karaite and Samaritan Passovers are each one day long, followed by the six day Festival of Unleavened Bread – for a total of seven days.

According to Exodus 12:3 the Passover lamb must be separated on the "10th day of the Month" therefore, a full cycle of the moon must be observed to determine when to hold the Passover. It cannot merely be held on the first full moon after the Spring Equinox. It must be observed after the first New Moon after the Equinox, then count 10 days, and 14th, and 15th, then always the Passover falls on the Full Moon.

Article source: http://en.wikipedia.org/wiki/Passover