Post History

Based on several Shiurim I've heard personally from Rabbi Yochanan Zweig.

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# What is עצם היום?

The Torah opens up with the beginning of Creation. In Bereishis 1:5, the Torah writes:

>וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם וְלַחֹשֶׁךְ קָרָא לָיְלָה‏
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>G-d called the light "Day," and the darkness He called "Night."

So the Torah expressly defines יום as the light created on the first day. 

What was this light? Chagigah 12a.9 describes it as follows:

>א"ר אלעזר אור שברא הקב"ה ביום ראשון אדם צופה בו מסוף העולם ועד סופו
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>R' Elazar said, "The light which Hashem created on the first day — Adam saw with it from the end of the world to its end."

As the Gemara goes on to describe, this special light was hidden away for future generations.

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# Revealing the Light

It stands to reason, then, that when the Torah elsewhere references יום, it's referring to something to do with this light. In the expression בעצם היום הזה, one might translate it not as "in the middle of the day," but rather _with the essence of the day_. That is, in the various instances where בעצם היום הזה is employed, Hashem revealed the light of Creation momentarily, for whatever purpose it was needed at that moment.

How would this have been employed in the three instances Rashi cites? Well, have you ever looked at the sun on a cloudless day? You know how blinding that is? Now amplify that by several orders of magnitude. Hashem's revealing the supernal light in these instances was effectively to blind anyone who would stop Noach from entering the Teivah,[^1] Bnei Yisrael from leaving Mitzraim, or Moshe from ascending Har Nevo. 

Avraham's Bris was different. The light wasn't needed to stop anyone from preventing the Bris from taking place. Its goal was instead to heal Avraham. Since Rashi is only discussing the three locations in which the light was employed to blind those attempting to stop an event from taking place, he doesn't discuss Avraham's Bris Milah; it's irrelevant to the point he's making.

[^1]: For those familiar with _The Little Midrash Says_, you may recall that Hashem sent lions and bears to stop them from destroying the Teivah. How does that fit with Hashem using the light? Rabbi Zweig explained by noting that Rashi doesn't bring that up on v. 13; he only discusses that on v.16, once they're already in the Teivah. So Hashem employed the light to allow Noach entry, and the animals to prevent harm once he was already inside. One thing that he did _not_ address is why Hashem revealed the light specifically when he entered, and the animals specifically once he was already inside, as opposed to the opposite or simply using one method of defense.