The Shabbat before every Pesach is known as “Shabbat HaGadol,” or “The Great Sabbath.” The term originates from the time of the Rishonim. While it is not exactly clear as to why Shabbat HaGadol was given this name, some claim that it relates to the custom of the Rabbi to give a long or great speech to his congregants on the Shabbat preceding Pesach.
Others (cited at Shabbat 87b) claim that the name is a reference to the great miracle that Hashem performed on the 10th of Nissan prior to the Bnei Yisrael’s exodus from Egypt. On the 10th of Nissan, all of the Bnei Yisrael gathered the sheep which were to be slaughtered four days later as Korbanot. Throughout three Parashiyot, the Torah provides a perspective on the Egyptians’ relationship with sheep. In Parashat VaYigash, the Torah describes how shepherds were “To’eivat Mitzrayim,” an “abhorrence to the Egyptians” (BeReishit 46:34); in Parashat Va’Eira, Moshe describes sheep as “To’eivat Mitzrayim” (Shemot 8:22); and in Parashat Mikeitz the Torah relates how the Egyptians could not eat together with the Jews, as it was a “To’eivah” for the Egyptians (Bereishit 43:32). Rashi famously takes the approach that the abhorrence, To’eivah, to which the Torah refers is a direct outgrowth of the Egyptians’ deification of the sheep (Bereishit 46:34 s.v. Ki To’eivat Hu). The Bnei Yisrael’s slaughter and consumption of the Egyptian deity would be regarded as an abomination by the Egyptians. When we began preparing for the slaughter of the sheep, there was an Egyptian backlash. The Egyptian people became infuriated when they saw us tying down the sheep. Yet, a miracle occurred, and they did not attack the Bnei Yisrael. Therefore, the “Gadol” in Shabbat HaGadol refers to this great miracle.
A major question is raised as to why we celebrate this miracle on the Shabbat preceding Pesach as opposed to the 10th of Nissan. A common answer that is given is that Miriam died on the 10th of Nissan. Therefore, instead of celebrating on the day of the month in which the miracle took place, we celebrate on the day of the week in which the miracle took place -- Shabbat.
Others see the reason to the name Shabbat HaGadol as a reference to a Pasuk at the end of its associated Haftarah. At the end of the Haftarah (Sefer Mal’achi 3), the coming of Mashiach is referred to as "Gadol’: ’Hinei Anochi Sholei’ach Lachem Et Eiliya HaNavi Lifnei Bo Yom Hashem HaGadol VeHanorah”, “Behold, I will send the prophet Elijah to you before the coming of the great, and terrible day of the Lord” (3:23).
In this Perek, Mal’achi addresses the Jewish people who had previously committed a variety of egregious acts, including: sorcery, adultery, lying, cheating one’s laborers, abandoning the tithe and contributions to the Levi’im, and treating the widows, orphans, and strangers poorly. Mal’achi views the people as standing before God, and he wonders how they will ever face Him again. Hashem reminds them that if they turn back towards Him, He will turn back towards them, giving them bountiful rewards (3:7).
Furthermore, Mal’achi notices that the people have become skeptical of the concept of Sechar Ve’Onesh (reward and punishment). However, Mal’achi adamantly points out that a day will come when God will mete out appropriate punishments and rewards for everyone. That day is described with language of heat and fire, as Mal’achi portrays those who are evil being consumed: “Ki Hinei HaYom Ba Bo’er KaTanur, VeHayu Chol Zeidim VeChol Oseih Rishah Kash,” "For behold, that day is at hand, burning like an oven; all the arrogant and evildoers shall be straw" (3:19). Meanwhile, those people who have feared Hashem will be rewarded with all that they need, “KeEglei Marbeik,” "like stall-fed calves" (3:20).
The Haftarah closes with a reference to Eliyahu HaNavi, who, I”YH, will come to herald the redemption quickly in our days. Mal’achi portrays the redemption as imminent, thus giving the people a strong incentive to repent and serve God. This parallels the redemption that we recall during Pesach. Just as the Bnei Yisrael were redeemed from Egypt after generations of slavery, so too will we be redeemed I”YH, with the coming of Mashiach quickly in our days.