Passover is a spring festival which during the existence of the Temple in Jerusalem was connected to the offering of the "first-fruits of the barley", barley being the first grain to ripen and to be harvested in the Land of Israel.
Passover commences on the 15th of the Hebrew month of Nisan and lasts for either seven days (in Israel and for Reform Jews and other progressive Jews around the world who adhere to the Biblical commandment) or eight days for Orthodox, Hasidic, and most Conservative Jews (in the diaspora). In Judaism, a day commences at dusk and lasts until the following dusk, thus the first day of Passover begins after dusk of the 14th of Nisan and ends at dusk of the 15th day of the month of Nisan. The rituals unique to the Passover celebrations commence with the Passover Seder when the 15th of Nisan has begun. In the Northern Hemisphere Passover takes place in spring as the Torah prescribes it: "in the month of [the] spring" (Torah prescribes it: "in the month of [the] spring" (Exodus 23:15). It is one of the most widely celebrated Jewish holidays.
In the narrative of the Exodus, the Bible tells that God helped the Children of Israel escape from their slavery in Egypt by inflicting ten plagues upon the ancient Egyptians before the Pharaoh would release his Israelite slaves; the tenth and worst of the plagues was the death of the Egyptian first-born.
The Israelites were instructed to mark the doorposts of their homes with the blood of a slaughtered spring lamb and, upon seeing this, the spirit of the Lord knew to pass over the first-born in these homes, hence the English name of the holiday.
When the Pharaoh freed the Israelites, it is said that they left in such a hurry that they could not wait for bread dough to rise (leaven). In commemoration, for the duration of Passover no leavened bread is eaten, for which reason Passover is called the feast of unleavened bread in the Torah or Old Testament. Thus matzo (flat unleavened bread) is eaten during Passover and it is a tradition of the holiday.
The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar. Passover is a spring festival, so the 15th day of Nisan typically begins on the night of a full moon after the northern vernal equinox. However, due to leap months falling after the vernal equinox, Passover sometimes starts on the second full moon after vernal equinox, as in 2016.
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 was 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 celebrated as legal holidays and as holy days involving holiday meals, special prayer services, and abstention from work; the intervening days are known as Chol HaMoed ("Weekdays [of] the Festival"). Diaspora Jews historically celebrated the festival for eight days. Reform and Reconstructionist Jews and Israeli Jews, wherever they are, usually celebrate the holiday over seven days. The reason for this extra day is due to enactment of the ancient Jewish 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 attaches only to certain (major) sacred 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.
Karaites 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 Rabbinic 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.