The Three Pilgrimage Festivals, known as the Shlosha Regalim, are three major festivals in Judaism - Pesach (Passover), Shavuot (Weeks), and Sukkot (Booths) - when the Israelites living in ancient Israel and Judea would make a pilgrimage to Jerusalem, as commanded by the Torah. In Jerusalem, they would participate in festivities and ritual worship in conjunction with the services of the kohanim ("priests") at the Temple in Jerusalem.
After 410 years of slavery in Egypt, Moses told Pharaoh that he must "Let My People Go." Each time Pharaoh refused to release the Israelites, God brought another plague upon the Egyptians. The tenth and final plague was the death of firstborn children. In executing this plague, God passed through the land of Egypt, but "passed over" Jewish homes. Thus, the holiday is called Passover. The Israelites were then released from bondage. But the Israelites only truly became free when, 50 days after the Exodus, they accepted the Torah from God at Mount Sinai. Thus, the Jewish nation was born.
Sources in the Hebrew Bible
Book of Exodus: "Offer a sacrifice to Me three times each year. Keep the festival of Matzos [i.e Passover]......Three times each year, every male among you must appear before God the Lord..." (Exodus 23:14-17) and "Keep the Festival of Matzahs [i.e. Passover]... Three times each year, all your males shall thus present themselves before God the Master, Lord of Israel." (Exodus 34:18-23)
Book of Deuteronomy: "Safeguard the month of standing grain so that you will be able to keep the Passover to God your Lord, since it was in the month of standing grain that God your Lord brought you out of Egypt at night...Then count seven weeks for yourself. From the time that you first put the sickle to the standing grain, you must count seven weeks.”...Three times each year, all your males shall thus be seen in the presence of God your Lord in the place that He will choose: on the festival of matzahs, on the festival of Shavuot, and on the festival of Sukkot. You shall not appear before God empty-handed." (Deuteronomy 16)
Every year, on the 15th day of the Hebrew month of Nissan, Jews worldwide commemorate the birth of the Jewish nation by celebrating Passover. Part of the commemoration is to tell the Passover story.
Taking this into account as well as other special mitzvot (commandments) Jews must perform on Passover, a Passover Seder is held. During the Seder, Jews relive the story of the Exodus as if they themselves are going from slavery to freedom.
On Passover Jews celebrate more than just a historical event. They celebrate their freedom as Jews. Just as God redeemed their ancestors from bitter slavery, today God redeems them from evil and grants them moral freedom. Just as God granted the ancestors of the Jews the Ten Commandments, today God expects them to realize the purpose of having been the chosen recipients of these laws.