Passover, or Pesach in Hebrew, is one of the Jewish religion’s most holy and observed holidays. In Judaism, Passover commemorates the story of the Israelites’ departure from ancient Egypt, which appears in the Hebrew Bible’s books of Exodus, Numbers and Deuteronomy.
The way it is celebrated today is very different from how it was originally celebrated. Three Thousand years ago the religion varied from place to place, hints we can see in the Bible which is the best written account of this time. The different folk beliefs practiced were tied to springtime and the celebration of the new year according to the Jewish lunar calendar. The two most prominent were that of the herders/migrants (Pesach), and those that were settled and survived off of farming (Hag Hamatzot).
An important milestone in this process took place in the reforms of the 16-year-old King Josiah in 622 BC when he commanded that the celebrations be taken out of the home and bring them to the Temple. It was then that he started renovating the temple. The the king commanded all the people, saying, Keep the passover unto the Lord your God, as it is written in the book of this covenant. It stipulated no sacrifices were to be made within their gates, but only on the altar in the Temple.
The feast of unleavened bread began the day after. All were commanded to avoid eating leavened bread for a week, The start of this practice was with the farmers. They also celebrated spring because it was then that new harvest would begin. The first grain to be ready for harvest was barley. Last year’s grain reserves had already been used and hunger took grip of the land.
The barley bread would have been unleavened, as the leavening agent used was a small portion that was saved from the previous batch. There would have been none left as there was no wheat left. Barley doesn't rise as much as wheat bread, and even if they used wheat flour it would have been flat and crispy, and matzoh was born.
Whilst there were many more changes that would make what and how Passover/Pesach is celebrated these are some of the basic influences.
Today, The Seder is a ritual performed by a community or by multiple generations of a family, involving a retelling of the story of the liberation of the Israelites from slavery in ancient Egypt. This story is in the Book of Exodus (Shemot) in the Hebrew Bible.