Since the time of our forefathers, they were practicing the Torah in all its nuances, but there was one area of halachah that remained undecided. They didn't know whether their practicing of the 613 mitzwoth exempted them from the 9 mitzwoth of the children of Noah. You might say that the 613 mitzwoth contain the 9 mitzwoth, true, but there are still very practical differences.
For example, he goes on to explain, once an animal has been slaughtered according to Jewish ritual law, the animal is now permissible for Jews to eat even if it is still kicking, whereas, since there is no concept of a proper way to slaughter animals for Bnei Noah, they must wait until the animal is dead, lest they transgress the prohibition of eating the limb of a living animal. (Whereas for Jews, the prohibition is only relevant before slaughter but not afterwards. Please note however that this is all a very technical example, but without the necesary information to derive practical halachah.) This safek, the doubt as to how to classify the children of Avraham, Yitzhak and Yaakov before Sinai, was actually at the root of the dispute between Yosef and his brothers. He told his father (Yaakov Avinu) that his brothers were transgressing the law of Bnei Noah by not waiting until the animal had died, whereas the brothers believed that since they kept all 613 mitzwoth of the Torah that they were exempt from the 9 mitzwoth of Bnei Noah.
This uncertainty created a major problem for our forefathers because it is clearly prohibited for a Bnei Noah to observe the Shabbath. Yet, if they wanted to properly keep the Torah, Shabbath observance is a major tenet of the 613 mitzwoth. What did they do as a result? They would observe the Shabbath flawlessly, except for a single transgression introduced in order to take into account this confounding uncertainty.
The Bnei Yissachar goes on to explain that the Rabbis all agree that according to the Torah, for a Bnei Noah there is only one valid way to purchase something. There is a disagreement as to what that one way is, but everyone agrees that there is only one way, it is either through meshicha pulling something into your possession, or through kesef, paying money for something. Rashi and Rabbeinu Tam disagree as to which way it is, but everyone agrees that one opinion is right and one wrong.
When HaShem commanded Bnei Yisrael to take the Pesah lamb, He tells Mosheh to tell them, "Pull and Take for you." The word used to Take here is a word that carries the connotation of purchasing with money. From the fact that HaShem, whose knowledge is without limit, commanded them to purchase the Pesah lamb both through pulling it into their possession, AND through paying money for it, Bnei Yisrael finally understood that they were no longer considered Bnei Noah. How so? Well, everyone agrees that Bnei Noah have only one way to purchase, meaning a second way would be irrelevant, and lest you say that they performed both kinds of purchasing just to be sure, there was no need 'to be sure' because HaShem doesn't have any 'doubt' about which kind of purchase is a valid purchase. (The Bnei Yissachar goes on to illustrate this chidush a little further, but this post is already complex enough.)
[To anyone who was brave enough to follow this far and hasn't gotten confused, I just wanted to reflect on this idea:] Stop and think about this. Their whole lives, for as many as five or six generations they had been guarding and keeping the Shabbath in every detail, except at one point each and every Shabbath, because of one uncertainty, they had to break the Shabbath somehow. No matter how small the transgression, it was an acknowledgement that they couldn't keep the Shabbath in its entirety. For more than four hundred years they ALMOST kept Shabbath. (that's more than 20,000 Shabbaths!)Can you imagine what it was like on Shabbath HaGadol, THE GREAT SHABBATH, that Bnei Yisrael kept Shabbath for the first time? Can you imagine the rejoicing and the singing?.. Can you imagine Seudah Shlishit, the third and final meal when they approached the end of Shabbath and they could hardly believe that this was it, this was their first Shabbath ever.
Shabbath is tantamount to the Beit HaMikdash. What Shabbath is to time, the Beit HaMikdash is to space. The Torah speaks about the completion of the Mishkan and the completion of the Beit HaMikdash, but for me this is the first time I ever heard about the completion of Shabbath. And we know, that while we don't have the Mishkan nor the Beit HaMikdash, we still have Shabbath. Shabbath and the Jewish people have celebrated thousands of years together. Wherever there is a Jew, that's where Shabbath will be. Shabbath and the Jewish people are inseparable. We can't begin to imagine the rejoicing of that first union.
That's why it's called The Great Shabbath.
May we all merit to experience A Great Shabbath this week, and may it uplift us into Pesah like we never experienced it before. (This year Shabbath HaGadol runs right into the first night of Pesah, when one ends the other begins.)