The Shabbos that precedes Pesach is known as Shabbos HaGadol, the “Great Shabbos.” The Alter Rebbe, Rabbi Shneur Zalman of Liadi, author of the Tanya and founder of Chabad Chassidism, cites the following reason in his Shulchan Aruch1 for the name Shabbas HaGadol:

“The Shabbos before Pesach is called Shabbas HaGadol since a great miracle occurred on that day. For ... when the Jews took the lambs for their Paschal offerings on that Shabbos, the Egyptian firstborn assembled before them and inquired why they were doing so. The Jews responded: ‘This is our Paschal offering, for G‑d shall slay the Egyptian firstborn.’

“[Thereupon] the firstborn went to their fathers and to Pharaoh and demanded that they liberate the Jews. When they refused to do so, the firstborn declared war against the rest of the Egyptians and killed many of them. This is the meaning of the verse, ‘…Who struck Egypt through its firstborn....’2 It was instituted that this miracle be remembered in future generations on this Shabbos, which is therefore known as Shabbas HaGadol.

Many other momentous events that occurred on this day are mentioned by other Sages as appropriate explanations for the name Shabbas HaGadol. Yet the Alter Rebbe only provides the reason mentioned above. Why does the Alter Rebbe choose this one specific reason to the exclusion of all others?

He does so because this particular reason identifies so strongly with the concept of Shabbos as a whole. When this particular miracle occurred on Shabbos, it underscored the fact that this was not a common Shabbos, but a “Great Shabbos.”

This means the following: Our Sages state that after the Six Days of Creation, the world was “lacking nothing but repose. When Shabbos arrived, so did repose.”3

Time is just as much a creation as are all other created beings and entities, including space.4 Inasmuch as time and space are inexorably related, and prior to the world’s creation space did not exist, time did not exist as well.

During each of the Six Days of Creation, then, time was constantly being created anew. Just as during each of these six days new creatures came into being, so, too, during each of these days a different span and aspect of time came into existence.

This being so, it would seem that prior to the seventh day, the world lacked not only repose, it lacked the entire seventh day as well: the framework of time represented by the seventh day had yet to be brought into existence. Why do our Sages say that after the Six Days of Creation the world lacked nothing other than “repose”?

Clearly, then, Shabbos and repose are so closely intertwined that we cannot differentiate between the two; when repose was created, the time configuration of Shabbos was simultaneously created together with it.

All Six Days of Creation share a certain similarity. In the framework of time, all of them consist of past, present and future — the basic elements of time. What is novel about the framework of time during Shabbos is that while it retains time’s basic dimension, it is not in a state of flux, but in a state of repose: it transcends past, present and future. During Shabbos, time is transformed and elevated to a level that transcends change.

A similar transformation occurred on Shabbas HaGadol. The firstborn of Egypt, the mightiest5 Egyptians and the greatest oppressors of the Jewish people, were suddenly and miraculously transformed by G‑d into people who — while remaining Egyptians — took up the Jewish cause. The similarity of this to time, which on Shabbos retains its basic composition yet at the same time transcends it, is striking.

This intrinsic connection between the Shabbos day and the miracle of Shabbas HaGadol is why the Alter Rebbechooses this as the sole reason why the Shabbos before Pesach is called Shabbas HaGadol.

Based on Likkutei Sichos, Vol. XVII,pp. 57-62.