And this day shall become a memorial for you, and you shall observe it as a festival for the L-RD, for your generations, as an eternal decree shall you observe it. For seven days you shall eat unleavened bread, but on the first day you shall remove the leaven from your homes ... you shall guard the unleavened bread, because on this very day I will take you out of the land of Egypt; you shall observe this day for your generations as an eternal decree. - Exodus 12:14-17

Of all the Jewish holidays, Pesach is the one most commonly observed, even by otherwise non-observant Jews. According to the 1990 National Jewish Population Survey (NJPS), more than 80% of Jews have attended a Pesach seder.

Pesach begins on the 15th day of the Jewish month of Nissan. It is the first of the three major festivals with both historical and agricultural significance (the other two are Shavu'ot and Sukkot). Agriculturally, it represents the beginning of the harvest season in Israel, but little attention is paid to this aspect of the holiday. The primary observances of Pesach are related to the Exodus from Egypt after generations of slavery. This story is told in Exodus, Ch. 1-15. Many of the Pesach observances are instituted in Chapters 12-15.

The name "Pesach" (PAY-sahch, with a "ch" as in the Scottich "loch") comes from the Hebrew root Peh-Samech-Chet, meaning to pass through, to pass over, to exempt or to spare. It refers to the fact that G-d "passed over" the houses of the Jews when he was slaying the firstborn of Egypt. In English, the holiday is known as Passover. "Pesach" is also the name of the sacrificial offering (a lamb) that was made in the Temple on this holiday. The holiday is also referred to as Chag he-Aviv, (the Spring Festival), Chag ha-Matzoth, (the Festival of Matzahs), and Z'man Cherutenu, (the Time of Our Freedom) (again, all with those Scottish "ch"s).

Courtesy of www.jewfaq.org