Bo: Rashi's choice of midrashim

I am short on time this week, but I answered a question on Mi Yodea about Rashi's choice of midrashim in parashat Bo. I will reprint the question and my answer here.

The question:

Why does Rashi on Shmos 12 (41) not quote the Sifrei which he does quote on Devorim 32 (48)?

In relation to the words בְּעֶצֶם הַיּוֹם הַזֶּה and the Exodus from Egypt, the Sifrei there says,

“Scripture states, “On that very day, the Lord brought [the children of Israel] out [of the land of Egypt]” (Exod. 12:51). The Egyptians said: “We swear by such and such, that if we notice them about to leave, we will stop them! And not only that, but we will take swords and other weapons, and kill them!” So, the Holy One, Blessed is He, said: “I will bring them out in the middle of the day, and let anyone who has power to prevent it, come and prevent it!”

This would seem to be a relevant quote on Shmos 12 (41).

My answer:

Rashi on Chumash channels midrashic works, selecting those midrashim that fit his stated criteria and reworking them to form a commentary.

Rashi has favored midrashim on different chumashim.

On sefer Shemot, he channels the Mechilta, which is a midrash composed on Shemot. Indeed,follow your link to Shemot 12 and see how many of his sources are from the Mechilta. This forms the main body of his commentary, will only occasional digressions to bring in salient points from other sources.

On sefer Devarim, he channels the Sifrei, which is a midrash composed on Devarim. Indeed, follow your link to Devarim 32 and see how many of his sources are from the Sifrei. This forms the main body of his commentary, will only occasional digressions to bring in salient points from other sources.

The citation from the Sifrei in question reads:

וידבר ה' אל משה בעצם היום הזה: בשלשה מקומות נאמר בעצם היום הזה, נאמר בנח (שם ז, יג) בעצם היום הזה בא נח וגו', במראית אורו של יום, לפי שהיו בני דורו אומרים בכך וכך אם אנו מרגישין בו אין אנו מניחין אותו ליכנס בתיבה, ולא עוד אלא אנו נוטלין כשילין וקרדומות ומבקעין את התיבה. אמר הקב"ה הריני מכניסו בחצי היום, וכל מי שיש בידו כח למחות יבא וימחה. במצרים נאמר (שמות יב, נא) בעצם היום הזה הוציא ה', לפי שהיו מצרים אומרים בכך וכך אם אנו מרגישין בהם אין אנו מניחים אותם לצאת, ולא עוד אלא אנו נוטלין סייפות וכלי זיין והורגין בהם. אמר הקב"ה הריני מוציאן בחצי היום וכל מי שיש בו כח למחות יבא וימחה. אף כאן במיתתו של משה נאמר בעצם היום הזה, לפי שהיו ישראל אומרים בכך וכך אם אנו מרגישין בו אין אנו מניחין אותו, אדם שהוציאנו ממצרים וקרע לנו את הים והוריד לנו את המן והגיז לנו את השליו והעלה לנו את הבאר ונתן לנו את התורה אין אנו מניחין אותו. אמר הקב"ה הריני מכניסו בחצי היום וכו':

And the Lord spoke to Moses on that very day: Heb. בְּעֶצֶם הַיּוֹם הַזֶּה. In three places Scripture employs the phrase: בְּעֶצֶם הַיּוֹם הַזֶּה [which has the meaning, “at the strongest light of the day”]. First, regarding Noah, Scripture states,“On that very day (בְּעֶצֶם הַיּוֹם הַזֶּה) Noah entered [… the ark]” (Gen. 7:13), which means in the glare of full daylight. Noah’s contemporaries said: “We swear by such and such, that if we notice him about to enter the ark, we will not let him proceed! Moreover, we will take axes and hatchets and split open the ark!” So the Holy One, Blessed is He, said: “I will have Noah enter at midday, and let anyone who has the power to prevent it, come and prevent it!” Second, regarding Egypt, Scripture states, “On that very day, the Lord brought [the children of Israel] out [of the land of Egypt]” (Exod. 12:51). The Egyptians said: “We swear by such and such, that if we notice them about to leave, we will stop them! And not only that, but we will take swords and other weapons, and kill them!” So, the Holy One, Blessed is He, said: “I will bring them out in the middle of the day, and let anyone who has power to prevent it, come and prevent it!” Likewise here, regarding Moses’ death, Scripture states,“on that very day (בְּעֶצֶם הַיּוֹם הַזֶּה) .” The children of Israel said, “We swear by such and such, that if we notice Moses [ascending the mountain to die], we will not let him do so! The man who brought us out of Egypt, divided the Red Sea for us, brought the manna down for us, made flocks of quails fly over to us, brought up the well for us, and gave us the Torah-we will not let him!” Thereupon, the Holy One, Blessed is He, said: “I will have Moses ascend the mountain [to his resting place] in the middle of the day!” - [Sifrei 32:47]

It appears in Sifrei on this verse at the end of Haazinu (in Sefer Devarim), but serves to analyze three pesukim throughout Torah which contain this phrase -- one in parashat Noach (sefer Bereishit), one in Bo (sefer Shemot), and one in Haazinu. And Rashi cites the local midrash on the local verse.

Meanwhile, in parashat Bo in Shemot, Rashi cites from his go-to midrash for Sefer Shemot, the Mechilta, and writes:

ויהי מקץ שלשים שנה וגו' ויהי בעצם היום הזה: מגיד שכיון שהגיע הקץ לא עכבן המקום כהרף עין, בחמישה עשר בניסן באו מלאכי השרת אצל אברהם לבשרו, בחמישה עשר בניסן נולד יצחק, ובחמישה עשר בניסן נגזרה גזירת בין הבתרים:

It came to pass at the end of four hundred and thirty years, and it came to pass in that very day: [This] tells [us] that as soon as the end [of this period] arrived, the Omnipresent did not keep them [even] as long as the blink of an eye. On the fifteenth of Nissan, the angels came to Abraham to bring him tidings. On the fifteenth of Nissan Isaac was born; on the fifteenth of Nissan the decree of “between the parts” was decreed. — [from Mechilta]

To restate your question with this new information, you are asking why, if Rashi finds the midrash in the Sifrei relevant to Devarim, and that Sifrei is also commentary on the pasuk in parashat Bo, then why does Rashi in parashat Bo not make use of the same midrash and the same explanation of the phrase?

And, I think the answer is that Rashi does not see the need to be consistent and always proffer the same consistent explanation across Tanach. Rather, his aim is to present a midrashically grounded traditional approach that explains salient points of the text, and particularly those which address issues of peshat or make sense of irregular phrases in a way that works well in the overarching context. In order to do this, he reworks classical works of midrash.

Here, local to parashat Bo, he already had a midrash from the Mechilta which addressed the phrase בעצם היום הזה, and one which fit into his framework of other midrashim from the Mechilta. He therefore had no need to look for a non-local midrash to explain the phrase. This even though he used that non-local midrash elsewhere, in a place where it was, in turn, local.

Note: I think the above answer is correct and answers not just this question but a plethora of other questions. However, an interesting related question is why, in parashat Noach, Rashi turned to this Sifrei and not to the parallel midrash (with the same idea as presented in the Sifrei) from Bereishit Rabba:

בעצם היום הזה בא נח אמר רבי יוחנן, אמר הקדוש ברוך הוא: אם נכנס נח לתיבה בלילה, עכשיו יהיו כל דורו אומרים כך: לא היינו יודעים בו, ואלו היינו יודעים בו, לא היינו מניחין אותו ליכנס! אלא בעצם היום הזה בא נח, דרגיש ליה ימלל.