The Shabbos before Pesach is called the 'Great Shabbos' (Heb. 'Shabbos Hagadol') on account of the great miracle that happened on that day. The Pesach sacrifice in Egypt was purchased on the tenth, and that year the exodus from Egypt was on the fifteenth, as is related in Shabbos 87.1. This means that the Shabbos before Pesach occurred on the tenth, and each took a lamb for his Pesach sacrifice and tied it to the leg of his bed. The Egyptians asked: 'What are you doing?', and they responded 'The Holy One Blessed be He commended us to slaughter it as a sacrifice. Now the Egyptians worshiped the lamb, and this set their teeth on edge, but they remained silent. Furthermore, the Israelites informed them the Holy One Blessed be He will [in a future plague] kill their firstborn, and this prompted the firstborn Egyptians to enter into hostilities with Pharaoh and their fathers, resulting in an internal war. This is the meaning of the verse "To strike the Egyptians through their firstborn, as is explained in the Midrash. It is due to this miracle [that the Egyptians entered into internal strife instead of attacking the Israelites] that gave rise to this Shabbos being called the 'Great Shabbos'.
Now if you should ask: 'What does the Shabbos have to do with this? let the commemoration always be on the tenth of the month' - know that there is an answer, which is that the tenth is also the anniversary of Miriam's [Moshe's sister] death, and therefore was not chosen to be a day of rejoicing. Furthermore, the tenth of this month was also the day when the Israelites crossed the Jordan upon entry into the Land of Israel, as is written in Yehoshua. Since a miracle accompanied this crossing, one might assume that any commemoration on the tenth is in memory of this miracle. Therefore habbos was set aside for commemoration of the miracle in Egypt, for the tenth did not fall out on Shabbos when the Israelites crossed the Jordan ('Bach').
If you will say to this: 'Why is it a problem if people connect the commemoration [if held on the tenth] to the crossing of the Jordan [it was also a great miracle]? The answer is that these days are set aside to commemorate the miracles of the Exodus, which were greater that those of the Jordan [and it would not be proper to have the day associated in people's minds with smaller miracles].
Moreover, there are those who write that this Shabbos was intricately connected with the miracle, for the Egyptians knew that the Israelites observed Shabbos, and that prompted the question 'What do you need with a Lamb on Shabbos [why are buying it today]?, to which they responded the reponse we have mentioned ('Levush'). Additionally, when they tied to lambs to their bed post they did an act [tying] normally be forbidden on Shabbos, and the Egyptians then asked after their behavior, to which they responded what I have mentioned ('Prisha').
One can also point out that in truth Shabbos depends on Pesach and Pesach depends on Shabbos, for Shabbos testifies to G-d's creation of the world, and Pesach testifies to G-d's divine providence over and love for the Jewish People. One without the other would not suffice, as is understood [in religious philosophy]. Therefore immediately after the Exodus from Egypt the Holy One Blessed be He gave us the commandment of Shabbos, and He did not wait until the giving of the Torah. This is because Pesach requires the ideology of Shabbos, and Shabbos requires the ideology of Pesach. This is why this Shabbos is called the 'Great Shabbos', for through this Shabbos the Holy One Blessed be He is extolled to a greater extent, for besides it testifying to His creation of the world, it also clarifies that His Divine Providence exists in the world. We are also raised through this, as we recognize that He chose us from among the other nations. (For more on this topic see what I have written in chapter 242.)
There is a custom on this Shabbos to refrain from the usual recitation of 'Borchi Nafshi'. Many read a portion of the Haggadah from 'Avadim Hyeenu' ('Our fathers were slaves...') until 'L'Chaper Al Kol Avonosainu' ('...to atone for all our sins.'). Some object to this custom, for the Braisa [in the Talmud] explicitly clarifies the time for this recitation: "I would have thought [we should commemorate the Exodus] from the beginning of the month - therefore the Torah specifies 'on that day' ". This clarification is referring to the verse of 'Ve'Hegadeta L'Vincha' ('You should tell your children...') [which is the source of the commandment of retelling the story of the Exodus on the night of Pesach] (Gra).
Returning to a previous point, we have already noted that the laws and philosophy of Pesach are expounded on this Shabbos. Some write additionally that if the eve of Pesach should fall on a Shabbos these public discourses should take place on the previous Shabbos (Chok Yaakov). This point of view works well for those who understand that the discourses are on the laws of Pesach. In our communities, where everyone asks their specific questions from their Rabbi, and the public discourses are given over to other religious matters, there is no reason not to hold them on Pesach eve [if the calendar falls out so]. In any event, aside from the public discourses, the laws of Pesach are recounted in the liturgical poem ['Piyut'] of Shabbos Hagadol.
On Shabbos Hagadol, the Haftorah of "v'Aravah" [Malachi, 3.10] is only read when Pesach eve falls on Shabbos, since it is written therein: "Bring the whole of the tithes into the treasury [that you have been holding back]", and Pesach eve was the day on which it was commanded that all overdue tithes should be removed [Heb. 'Biyur'] from one's house on the fourth and seventh years of the 'Shmittah' cycle.
Others insist that the rule is the reverse: Since one cannot perform 'Biyur' on Shabbos [due to the restrictions of Shabbos] one does not read the Haftorah of "v'Aravah", and it is only on other years that "v'Aravah" is read as a commemoration (in the name of the Gra). This in fact is our custom, to read "v'Aravah" on every Shabbos Hagadol, except when Pesach eve falls on Shabbos, when the regular Haftorah of the weekly Torah portion would be read.