What makes Shabbat haGadol so Gadol?

(3) Speak to all the congregation of Israel and say: On the tenth of this month, each one of you will take one sheep per family, one sheep per house.

In the Nisan when Israel went out from Egypt: on the fourteenth day they slaugthered their paschal-lambs, and on the fifteenth they went out, and that evening the firstborn were stricken. The evening after they left — does such a stupid idea even enter your mind!? No, rather on the evening before they left the firstborn were stricken. And that very day was a Thursday.

(1) The Shabbat that is before Passover people are accustomed to call Shabbat haGadol [the Great Shabbat], but they do not know for what reason there is no greater one of all the Shabbatot but this one in Nisan, for Israel went out from Egypt on a Thursday, as it says in Seder Olam, and thus took their paschal-lamb on the Shabbat before Passover. Israel said, "Hey, if we sacrifice the abominations of Egypt, will they not stone us?" The Holy Blessed One said to them, "Now you will see the miracle that I will do for you." So they went, and each took their Passover-lamb to be guarded for four days, until the fourteenth day. When the Egyptians saw this, they wanted to rise up and take vengeance on it, but their bowels were stretched and their fire was going out and they were sentenced to torture and horrible and constant and bitter sicknesses, and they could not injure Israel at all. And because of the miracles that were done for Israel on that Shabbat that was before Passover, thus we call the Shabbat before Passsover Shabbat haGadol. From the mouth of Rav Avraham whose soul is in paradise. (Rashi, Troyes, France, 1040—1105)

(1) The Shabbat before Passover is called "Shabbat HaGadol" (The Great Shabbat). And the reason is because a miracle occurred during the Exodus from Egypt. On the 10th [they took a sheep] as it says: "On the tenth of this month you shall take for yourselves a sheep into your homes." And the year that the Exodus took place was on a Thursday as we see in Seder Olam, and therefore the "10th of the month" was Shabbat, and every Jew took a sheep as a Paschal offering and tied it to their bedposts. And the Egyptians asked them "Why is this so?" And they responded: "To slaughter for a Passover offering for haShem." And they got upset that they were going to slaughter their gods, but they could not say anything due to the miracle. And thus it is called Shabbat HaGadol. (the Ba'al haTurim, Jacob ben Asher, western Europe, 1269—1343)

(4) And they will be pleasing to haShem, the grain-offerings of Judah and Jerusalem, as in days of old and years of yore.

(23) Hey, I send to you Elijah the prophet, before the coming of the day of haShem, the great and awesome.

Hinei anokhi sholéaḥ lakhem et Eliyyah ha-navi lifnei bo yom A-d-o-n-a-i ha-gadol v-ha-nora.

(2) Adonoy return us. Because he concludes with words of reproof, he had to repeat the preceding verse again, and so did Isaiah, Malachi, and Kohelet.

(7) And the Shabbat before Passover is called Shabbat haGadol, and many words have come about this, but none explain it except that we read as a haftara that day "And they will be pleasing to haShem" (Malachi 3:4-24) and there it says (Malachi 3:23) "before the coming of the great [haGadol] and the awesome day of haShem:" For this reason it is called Shabbat haGadol. And the explanation that the Exodus from Egypt was called great on account of the greatness of God's awesome acts that God did and acted when they left Egypt, and in the future, in the world to come, Israel will aquire even higher levels, and the Holy Blessed One will do even greater deeds than all the great deeds, and even more awesome than all the awesome deeds. And as Israel merited the level of the Exodus from Egypt when great deeds were done with them, they will merit an even higher level and and deeds will be done in the world to come. And thus the Shabbat before Passover is called Shabbat haGadol, for every Shabbat is a source and remembrance of the world to come which is called Shabbat as well, and thus the Shabbat preceding Pesach is a remembrance of the great Shabbat that will be the "great and the awesome day of haShem."

...And thus for the matter of Shabbat haGadol, and look, it is made clear that the Shabbat before Passover we can call it Shabbat haGadol for Passover is great, as above, and the Shabbat relying on it is the Shabbat even more magnified and blessed, for it is all the more the great and awesome day, and understand well that this is correct and there is no doubt for understanding it. (the Mahara"l, Judah Loew ben Bezalel of Prague, c. 1520—1609)

(1) And this is the aspect of Shabbat HaGadol, which is the name of the Shabbat before Passover, on account of the miracle, etc. The essence of the miracles and wonders that took place during the Exodus from Egypt - which crumbled the battles and nullified nature and revealed His Blessed Providence to the world - all of it was drawn from the aspect of Shabbat, which is the Future World which is totally Shabbat. From there, Hashem Yisbarach drew, so to speak, divine providence from the ends of the earth, etc., corresponding to (Ezekiel 7:2) "an end! The end has come", etc., as is brought there in the lesson as seen above. The essence of the Exodus from Egypt is drawn from Shabbat, which is the aspect of the World to Come. Therefore, Pesach is also called Shabbat, as it is written (Leviticus 23:15) "And you shall count for yourselves, from the morrow of the rest day"; in other words, Pesach, which our Sages of Blessed Memory said (Menachos 65b) that the essence of the power of the miracle of Passover is drawn from Shabbat, as explained above. This is the aspect of the Shabbat that is before Passover, which is called Shabbat HaGadol because of the miracle. For all of the miracles of the Exodus from Egypt were drawn from the aspect of Shabbat, as explained above. Therefore it is called "Gadol" because of the miracles which were called the aspect of Gadol (great), as it is written (II Kings 8:4) "Please tell me all the great things that Elisha performed". As Rabbeinu z"l wrote there in the lesson as seen above, 'And 2 tears went down into the great sea', see there. (R. Nachman of Breslov, 1772—1810)