The Shabbat before Passover is called "Shabbat HaGadol" (The Great Shabbat). The main reason for this is that on this day, a great miracle occurred in Egypt. The Jewish people were commanded to take a lamb and keep it in their homes until it was slaughtered on the eve of Passover. The Jews told their Egyptian neighbors that they were going to sacrifice the lamb to G-d, so that G-d would kill the firstborn of Egypt. When the Egyptians' firstborn heard this, they demanded that the Egyptians free the Jews. When they refused, the firstborns fought against them and killed many of them. It is said, "To Him Who smote the Egyptians with their firstborn, for His kindness is eternal." (Tehillim, 136:10)
The Enemy Becomes a Friend
That year, the 10th of Nissan fell on Shabbat, and so we mark this every year on the Shabbat before Passover. This requires an explanation: Why is the memory of the miracle commemorated on the weekly day it occurred, rather than on the monthly date (the 10th of Nissan), as all Jewish festivals are established? A simple reason for this is provided in the Shulchan Aruch: "Since Miriam died on the 10th of Nissan and a fast was established on it when it falls on a weekday." Yet, this reason seems somewhat secondary. We must say this miracle is especially tied to the essence of the day of Shabbat. On the verse, "A psalm...for the Shabbat day," (Tehillim 92:1) the Midrash says: "For the Sabbath day — to stop the harmful forces from the world so they do not cause harm, as it is written, 'And the wolf shall dwell with the lamb.'" The cessation of harmful forces can occur in two ways: 1) they remain harmful but do not cause harm, or 2) they cease being harmful and become friends and helpers. When the Midrash cites the verse, "And the wolf shall dwell with the lamb," it means the harmful forces transform into friends.
The Firstborn Fight
Indeed, this kind of cessation related to Shabbat will occur in the future, but it also happened on the first Shabbat, the Shabbat of creation, when the harmful forces did not cause harm. Similarly, this miracle of cessation occurred on Shabbat, the 10th of Nissan, before the Exodus from Egypt. At that time, Egypt was at the height of its opposition to G-d. The nine plagues that the Egyptians endured did not break their resistance. Then came Shabbat and caused the cessation of the harmful forces — the firstborn of Egypt, who symbolized the peak of the enemy's power, went out and fought on behalf of the Jewish people. The enemy itself fought for the liberation of the Jews from Egypt.
Strength in Exile
This is the meaning of the term 'Shabbat HaGadol' — the essence of Shabbat stands on this day with great power: the cessation of harmful forces at its highest level — not just the breaking of evil, but the transformation of evil itself into good, enlisting it to help holiness. This Shabbat gives strength to a Jew even during exile. Even though the Jewish people are in exile, in a state of darkness that is doubly oppressive (like the situation of the Jews on the 10th of Nissan before the redemption), the Jew is empowered to serve G-d with all his might, influencing one Jew after another, until he succeeds in influencing the nations of the world, who will also assist the Jewish people. And even their 'firstborns,' symbolizing their strength and power, will strongly demand good for the Jewish people.
(the Rebbe, Likkutei Sichot, Volume 17, translated from Sichat HaShevua)
