The Shabbos that precedes the festival of Pesach is known as Shabbos HaGadol , the “Great Shabbos,” commemorating the miracle that transpired with the Jewish people on the tenth of Nissan , which that year fell on a Shabbos, just prior to the Exodus.

Many Rabbis have asked1 why the commemoration of this event was assigned to the day of Shabbos, rather than to the tenth of the month, as is the case with all other festivals, which are celebrated on the anniversary of the date of the month, and not on the anniversary of the day of the week.

The relationship of Shabbos HaGadol to the day of the week will be understood if we consider the reason that all other festivals are celebrated according to the day of the month.

The difference between the days of the week and of the month is that the days of the week are bound up with the Seven Days of Creation and the world itself — “For in six days, G‑d made the heavens and the earth, and on the seventh day He ceased from work and rested.”2

The concept of a “month,” however, is a later addition. As our Sages state:3 “When G‑d chose His world, He [then] established within it new months.” Thus, months are not bound up with creation and nature itself, but with G‑d’s choosing, and as such denote an additional quality that He imbued “within His world.”

The difference between days of the week and days of the month is also connected to the very nature of weeks and months: weeks, composed as they are of a seven-time cycle of day and night, are related to the sun, while the days of the Jewish month are related to the cycles of the moon.4

The illumination of the sun is constant, like the unchanging quality of nature, while moonlight, which waxes and wanes, is always in a state of flux. Thus, the cycles of the moon are symbolic of an addition to creation — the miracles that transcend nature.

Since all Jewish holidays and festivals celebrate revelations of G‑dliness that transcend nature,5 it follows that they are all celebrated on the day of the month on which they first occurred.

The reason why the miracle of Shabbos HaGadol is not celebrated in accordance with the day of the month but in accordance with the day of the week will be understood accordingly.

The Alter Rebbe explains the miracle of Shabbos HaGadol in the following manner:6 “When the Jews took their Paschal lamb on that Shabbos, the first-born Egyptians gathered round and asked them why they were doing so. They told them it was for the festival of Pesach, at which time G‑d would smite all the Egyptian firstborn.

“The firstborn thereupon went to their parents and to Pharaoh and asked that they send the Jews out of Egypt. When they refused to do so, the firstborn engaged them in battle and killed many of them. Thus the verse states:7 ‘Who struck Egypt through its firstborn.’ ”

At first glance, the entire incident seems to be a natural event: After having experienced the first nine plagues, it is no wonder that the firstborn believed they were about to be smitten. Thus, when their elders refused to allow the Jews to leave, it was quite natural that they rose up and killed many of them.

Thus, the miracle of Shabbos HaGadol lay not in an overpowering of nature with a revelation of the supernatural, but merely that nature itself underwent a change: The Egyptian firstborn — until then the most powerful of the oppressors8 — did all they could to have the Jews leave Egypt.

The miracle of Shabbos HaGadol is therefore linked to the days of the week — the course of nature — for it involved a change within nature itself. This is why this miracle is always celebrated on Shabbos rather than on the tenth of Nissan , for it specifically relates to days rather than months.

Based on Likkutei Sichos, Vol. XXVII, pp. 44-46