This week’s Shabbat - Parshat Metzorah - is known as Shabbat Hagadol; it is a commemoration of the Shabbat before the Exodus, in which the Jews were told to take a lamb in preparation for the Paschal sacrifice. Though the lamb at the time was a major Egyptian deity, the Jews were able to perform this mitzvah (commandment) unhindered – a truly wondrous miracle!
Interestingly, the Shabbat before the Exodus - when the Jews were commanded to begin preparing their lambs for the Paschal sacrifice - fell on the Hebrew date of the 10th of Nissan. The Paschal lamb would be sacrificed on the afternoon of the fourteenth; and the Exodus would take place the night of the fifteenth.
(It is astonishing to realize that there were 9 plagues up until this point, including blinding locust swarms, water turning to blood, and hail of fire and ice pouring down upon Egypt – and yet, the anniversary of those plagues is hardly celebrated (if even known at all). It appears that the ability of the Jews to take the pagan representation of the Egyptian society in full view, and prepare it for sacrifice four days later, may have been a greater miracle than anything that had occurred up until that moment.)
As mentioned, the date that the Jews were to begin preparing the Pesach (Paschal) sacrifice was Shabbat, the 10th of Nissan. Much later on (approximately 40 years later!), the Torah states in Bamidbar 20:1:
“The Israelites arrived in a body at the wilderness of Zin on the first month [Nissan], and the people stayed at Kadesh. Miriam died there and was buried there.”
The Torah explicitly says that Miriam died during the first month, Nissan; the Torah does not, however, state the date of her death. Targum Yonasan, ad. loc., comments that Miriam in fact died on the 10th day of the month.
Magen Avraham (on Shulchan Aruch Orach Chaim, Chapter 430’ para. 1’) comments that based on the above chronology, we can understand the answer to a rather perplexing issue—namely, general Jewish custom is not to follow the day of the week an event took place, but rather the Hebrew date for said event. Hence, we celebrate Pesach on the 15th of Nissan, whether or not the 15th falls on a Thursday (the Jews left Egypt on a Thursday), or any other day of the week. If that is the case, why is Shabbat Hagadol different, and associated not with the date of the month that the miracle took place (the 10th of Nissan), but with the day of the week that the miracle occurred (the Shabbat before Pesach, no matter the date)?
Magen Avraham explains based on the Targum Yonason quoted above, that the establishment of Shabbat Hagadol on the 10th of Nissan would have been impossible, for that day was destined to become the anniversary of Miriam’s death! Therefore, rather than commemorate the 10th of Nissan as a celebratory day, which would eventually be a slight to the righteous Miriam, custom became to recall the miracle of the preparing of the Paschal sacrifice in Egypt on the Shabbat before Pesach, even if the date of that Shabbat happens not to be the 10th of Nissan.
Interestingly, Rama writes in Orach Chaim Chapter 299 that there was a custom to drink fresh water immediately after Shabbat ends, for the Well of Miriam disperses into the waters of the world at that moment, and if one happens to drink that water, all of his\her maladies will be healed. (Rama comments that he himself did not ever see this custom practiced.)
Rama does not offer a reason or clue as to how or why this dispersal of the Well of Miriam takes place after Shabbat; it would seem that there is something about Shabbat that acts as a replication of Miriam’s life force; as such, on Shabbat the Well is understood to be functioning much the same as when Miriam was alive – it is only after Shabbat ends that the Well disappears; just like after Miriam's death, the rock that had given the Jews water in the desert dried up (see Taanit 9A).
Thus, it is not simply that we moved the date of commemoration of the taking of the Paschal Lamb — in full view of the Egyptians — from the 10th of Nissan to the Shabbat immediately prior, because of Miriam’s death taking place on that date; rather, the Shabbat serves as a conduit to recreate the merit of Miriam — as evidenced by the waters of her well only scattering after Shabbat ends — and therefore, despite the fact that the original date of this miracle would become the date of Miriam’s death, by combining the remembrance of that miracle with the holiness of Shabbat day, we are in fact comforted over her loss, as we pray for the return of all of Israel’s departed speedily in our days.