Historically, together with Shavuot ("Pentecost") and Sukkot ("Tabernacles"), Passover is one of the Three Pilgrimage Festivals (Shalosh Regalim) during which the entire population of the kingdom of Judah made a pilgrimage to the Temple in Jerusalem.The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar.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.In Jewish practice, one or two festive Seder meals – first two nights; in the times of the Temple in Jerusalem, the Passover sacrifice.In Samaritan practice, men gather for a religious ceremony on Mount Gerizim that includes the ancient lamb sacrifice (7th day) Pesah, Pesakh), is an important, biblically-derived Jewish holiday.Jews celebrate Passover as a commemoration of their liberation by God from slavery in ancient Egypt and their freedom as a nation under the leadership of Moses.
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.
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 the first month, on the fourteenth day of the month at dusk is the LORD's Passover.
And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD; seven days ye shall eat unleavened bread.
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).