The Talmud, Shabbat 87b helps us calculate when the first Shabbat HaGadol (Great Shabbat) took place. According to the Talmud, the 15th of Nisan, the day that B’nai Yisrael left Egypt, was a Thursday. This means that if we count backwards, the Shabbat before B’nei Yisrael celebrated Pesach in Egypt was on the 10th of Nisan.
The 10th of Nisan is also the famous date when B’nai Yisrael were told (Shmot 12:3) to “take for themselves-each man- a lamb or kid for each father’s house, a lamb or kid for the household.”
Chizkuni points out that since B’nai Yisrael observed this first mitzvah of taking the lamb into their homes, the Shabbat is called great.
Tosafot brings a Midrash to explain why this Shabbat was so great and miraculous:
When B’nai Yisrael took their lambs on that Shabbat, the firstborns of the nations asked them what they were doing. B’nai Yisrael answered, “The animals are being prepared for a Pesach offering to God, who will kill the firstborns of Egypt.” The firstborns went to their fathers and told them to ask Pharaoh to send B’nai Yisrael out of Egypt. However, their fathers did not want to comply. The firstborns then waged war against the other Egyptians, and suffered massive losses.
Chizkuni adds that the zodiac sign for the month of Nisan is Aries (a lamb). It was a sign that served as a deity of good fortune for the Egyptians and would now become the reminder of good fortune for B’nai Yisrael. By adopting this symbol and slaughtering a lamb as a sacrifice instead of worshipping it, B’nai Yisrael demonstrated their faith in God. They thereby neutralized any good fortune this constellation had ever had portended for the Egyptians.
By taking the lamb into their homes on this Shabbat, B’nai Yisrael made the commitment to give up idol worship and trust solely in God. This was the greatest thing that they could have done and this jump-started the process of the geula, the redemption from Egypt.