Maran, in Shulhan Aruch, discusses the special Shabbatot that precede Pesah and the various Halachot that apply on those days. On Shabbat Shekalim, we take out an extra Sefer Torah and read the section of Shekalim, just as on Shabbat Zachor, the Shabbat before Purim, we read the section of Zachor, and we read special portions from the Torah on Shabbat Para and Shabbat Ha’hodesh. Interestingly, however, Maran also makes mention of the Shabbat immediately preceding Pesah, which is called Shabbat Hagadol ("The Great Shabbat") because of the miracle that occurred on this Shabbat. Maran makes mention of this Shabbat, even though there are no specific Halachot associated with this Shabbat. There is no special reading from the Sefer Torah on this Shabbat, and there are no special prayers, and yet Maran made a point of mentioning in his Halachic code that this Shabbat is special and is called Shabbat Hagadol.
It is true that there are several customs observed by some communities on this Shabbat. For example, many synagogues read a special Haftara on this Shabbat, but this is not required according to the strict Halacha. Some Ashkenazim read portions of the Haggadah on Shabbat Hagadol, but this, too, is just a custom, and in fact the Vilna Gaon (Rabbi Eliyahu of Vilna, 1720-1797) was opposed to the practice.
The Hida (Rav Haim Yosef David Azulai, 1724-1807) suggested that perhaps the Halachic implications of the status of Shabbat Hagadol relates to greetings extended to one another on this Shabbat. Instead of greeting one another with the usual greeting of "Shabbat Shalom," the Hida suggested that perhaps one should instead extend the greeting, "Shabbat Hagadol Shalom," and the other should then respond, "Shabbat Hagadol Shalom U’meborach." Still, it remains unclear why Maran would make a point of mentioning Shabbat Hagadol if there are no actual Halachot that apply on this Shabbat.
Former Chief Rabbi Eliyahu Bakshi-Doron, in his work Binyan Ab, suggested that the practical significance of Shabbat Hagadol lies in the perspective it gives us on the preparations for Pesah. On the Shabbat before the Exodus, which was the 10th of Nissan, Beneh Yisrael were instructed to prepare a sheep for the Korban Pesah (paschal sacrifice) which was slaughtered four days later, on the 14th of Nissan. In full view of the Egyptians, who worshipped sheep as an Aboda Zara, Beneh Yisrael took a lamb and tied it to their bedpost, and they even explained to the Egyptians what they were doing, that this animal would be sacrificed to G-d. The Egyptians wanted to kill the people for desecrating their deity, but G-d performed a miracle and made them powerless to cause Beneh Yisrael any harm.
Rav Bakshi-Doron noted that this miracle differed fundamentally from the ten plagues. The ten plagues were brought directly by G-d, whereas the miracle of Shabbat Hagadol came about through the people’s courage. It was through their preparations for the Misva that a miracle happened. And the fact that G-d performed a miracle during the preparation for the Misva, four days before the actual performance of the Misva, is very significant. It demonstrates that even the preparatory stages have great value; that even the work we do in preparation for a Misva is laden with significance. And thus Maran made mention of Shabbat Hagadol in Shulhan Aruch, to teach us the importance of preparing for Pesah, by learning and teaching the Halachot and putting ourselves in the proper frame of mind for the holiday. The miracle of Shabbat Hagadol teaches us that it is not only the Misva itself that has value, but also the work we invest beforehand preparing ourselves for the performance of the Misva.