Divine design. The Shabbat before Pesach, we call it Shabbat HaGadol. Why do we call the Shabbat Shabbat HaGadol? The first fundamental belief is that G-d created the heavens and the earth. The reason is the great miracle that took place on the Shabbat before the Jewish people left Mitzrayim (Egypt). On that day, G-d told the Jewish people to take a sheep and tie it to their bedposts. Eventually, they would slaughter it four days later, right before they left. There was a miracle because the Egyptians worshipped the sheep. They asked the Jews, “What exactly are you doing with that sheep?” The Jews said, “We’re taking it, and we’re going to slaughter it and make a Korban (offering) to G-d.” Logic says the Egyptians sort of killed the Jewish people as a result. But G-d performed a miracle: the Egyptians either got a sickness in their bodies that left them unable to react, or they became afraid, and the Jewish people were saved from the revenge of the Egyptians that day.
Furthermore, the Aruch HaShulchan writes that on that day, a civil war broke out in Egypt between the firstborns and their parents. The firstborns believed there was going to be a Makkah (plague), and they didn’t want to die. So they pleaded with their parents, “Let the Jewish people go.” Their parents didn’t want to let them go. So the Pasuk (verse) says, “But there was a Makkah in Mitzrayim from the firstborns,” which happened on that day as well. Because of these miracles, we commemorate the day and call it Shabbat HaGadol because of the great Nes Gadol (great miracle) that happened on that day.
A question is asked. It’s true that when we left Egypt, the day we took the Korban Pesach (Passover offering) was a Shabbat, because we left Egypt on a Thursday and they took it four days before. There’s no question that during that year, it was a Shabbat. We even know the date: it was the 10th of Nisan. So the question is, why don’t we commemorate the date, the 10th of Nisan? Why don’t we commemorate the Shabbat before Pesach? We should commemorate the calendar date. For all our holidays, we don’t commemorate the day the event happened; we commemorate the date it happened. The date of that miracle was 10 Nisan, which happened to be a Shabbat that year. So we should commemorate 10 Nisan every year, whether it comes out on a Tuesday, a Thursday, or a Monday.
Many answers have been given to this. One answer is that on the 10th of Nisan, a tragedy happened to the Jewish people: we lost the great prophetess, Miriam HaNevi'ah. It wouldn’t be proper to establish a celebratory holiday on the 10th of Nisan, the day Miriam passed away. So we don’t commemorate it by the date. Another answer is that on the 10th of Nisan, a great miracle happened, but it had nothing to do with Egypt. In the times of Yehoshua, when they crossed the Jordan River, the Jordan River split on that day. That was a great miracle. If we commemorated the 10th of Nisan, people would not know which miracle we were commemorating. They would think we were celebrating the splitting of the Jordan. So, to avoid confusing it with the miracles I already mentioned, they commemorated it on the Shabbat before.
However, the Aruch HaShulchan then gives a beautiful answer as to why they established this commemorative holiday on the Shabbat before Pesach, and not on the 10th of Nisan. He says that Shabbat and Pesach go together; they’re linked. And we don’t want to separate Shabbat and Pesach. Why? In Judaism, there are two fundamental beliefs. There’s a creator: the world did not happen by itself. The second and equally important principle is Hashgacha Pratit: after G-d created the world, there is divine providence. He is involved in all the goings-on of the world. G-d forbid that we think He created the world and just left it to run on its own. There is active participation: G-d is involved every moment in the orchestration of events that take place in the world, from the biggest events to the smallest events.
We learn those two principles from Shabbat and Pesach. Shabbat reminds us that there’s a creator. Six days G-d created, and on the seventh day He rests. Pesach teaches us about divine providence: He knew exactly who the firstborns were; they died. G-d knew exactly who the good people were; they were saved. The Resha'im (wicked people) were punished. G-d knows exactly what’s going on at all times, to all people, at all moments on earth. Shabbat solidifies that there’s a Boreh (creator), and Pesach solidifies that there’s a mashgiach (overseer). One without the other does not work. You need both these principles to work together. Therefore, we wanted to commemorate Shabbat and Pesach together. We don’t commemorate this miracle on the 10th of Nisan; we commemorate it on the Shabbat before Pesach. So we can make a link between the two fundamentals of our belief system.
It should be noted that Shabbat was so important that when we came out of Egypt, G-d gave us Shabbat before the Ten Commandments were even given. You cannot wait for the Ten Commandments to give us Shabbat, because once we have a Pesach, Pesach cannot come without a Shabbat. So when we celebrate Shabbat HaGadol, both these concepts merge together: it is a day that is Shabbat and a day that commemorates the miracles of Mitzrayim. Therefore, it is a holy day. That could be why it’s called Chag (holiday), because of the great principles that the day represents.
Nonetheless, for us, there aren’t too many halachot (Jewish laws) that we have to know regarding this day, besides the fact that the Hida says it is customary to greet each other by saying Shabbat HaGadol Shalom, because Maran (Our Master) says we call the day Shabbat HaGadol, so you have to call it that. Instead of just saying Shabbat Shalom, we say Shabbat HaGadol Shalom, and we answer back, Shabbat HaGadol Shalom Umevorach. It is also the custom of the rabbi of the congregation to give a special derasha, called the speech of Shabbat HaGadol. That speech includes halachot that are necessary for the people to be prepared for the holiday, as well as divrei musar (ethical teachings) and Hitorerut (spiritual awakening), in order to prepare them mentally for the spiritual and hashkafic (worldview-related) angles of the great Chag.