The Background to the Passover Holiday from Teaching Jewish Holidays:

Leviticus 23:5-8 contains the biblical injunctions to celebrate the festival of Pesach: “in the first month, on the fourteenth day of the month, as twilight, there shall be a passover offering to the Lord, and on the fifteenth day of the month is the Lord’s Feast of Unleavened Bread.  You shall eat unleavened for seven days. The first day shall be for you a sacred occasion: you shall not work at your occupations. Seven days you shall make offerings by fire to Adonai. The seventh day shall be a sacred occasion: you shall not work at your occupations.”

Efforts to probe the origins of the Jewish holidays are in no way intended to denigrate the importance or the meaning of the festivals. Rather, they help to demonstrate the genius of the Jewish people who drew from their environment, transforming what might have begun as a pagan idea or practice into something with universal and timeless significance.

According to such scholars as Hayyim Schauss and Theodor Gaster, the modern festival of Pesach is a combination of a shepherd’s festival and an agricultural festival.  Leviticus 23 reinforces this idea.

It is possible that a Pesach festival had its earliest stirrings among semi-nomads who thousands of years ago wandered the desert and the semi-arid environs around Palestine.  Nisan was the month when sheep most often gave birth. These nomads came to observe a festival at the time of the full moon. Just before nightfall, a sheep or goat was sacrificed. The animal was then roasted, and the family ate a hasty meal so that all of the animal would be eaten before daybreak.  No bones of this sacrificial animal could be broken. Tent posts were daubed with the blood of the slain animal as an antidote to plagues, misfortune, and illness.

Perhaps the Feast of Unleavened Bread was a six or seven day festival marking the beginning of the spring harvest and the offering of the first sheaf of the newly cut barley to the priest as a sacrifice to God.  The elimination of chametz may have originally been precautionary so as not to infect the new incoming crop.  Or, it may have been a way of propitiating the priests and God so as to assume health and bounty.

As Judaism moved away from being agriculturally based, new interpretations and new customs were added to the Pesach ritual so that Jews living all over the world and in all ages could meaningfully celebrate Pesach.  The prototype Haggadah finds its way into the Mishnah as Tractate Pesachim.  By the end of the Talmudic period, its form and much of its content were as they are today.  It must have been widely accepted, because the Haggadah was included in the very first prayer book of Rav Amram in the eighth century, as well as in the prayer book of Saadia Gaon in Babylonia (tenth century).  Somewhere around the twelfth century, it began to be copied as a separate book. It attracted many commentaries, and became the favorite subject of Jewish artists who found the subject liberating.