In his Derech HaShem, Ramchal writes that all holidays are connected to the original day thousands of years prior when the events of that holiday first took place. That is to say that the same spiritual energies that were at play on the first Pesach, Shavuos, Succos, etc., have been at play ever since on those days as well. There are intrinsic spiritual energies linked to these various days.
As such, the seventh day of Pesach nowadays links back to the original seventh day of Pesach as well. The seventh day of Pesach was when the Jewish people experienced kriyas yam suf and first sang As Yashir in praise of, and thanks to, God. Today, over 3,000 years later, the Jewish nation still recites Az Yashir in thanks to God on the seventh day of Pesach. Why, then, do we not sing a full Hallel in thanks to God on this day as well?
The Gemara (Arachin 10a-b) states that the reason we do not recite a full Hallel on the latter days of Pesach is because no new korbanos are brought on those days (unlike Succos on which a new korban was brought throughout, thus requiring a full Hallel throughout as well). The Yalkut Shimoni, however, proposes a different answer entirely. It brings as the reason we do no recite Hallel on the latter days of Pesach the following posuk from Mishlei:
בִּנְפֹל אוֹיִבְךָ אַל־תִּשְׂמָח וּבִכָּשְׁלוֹ אַל־יָגֵל לִבֶּךָ׃
Rejoice not when thine enemy falleth, And let not thy heart be glad when he stumbleth;
The posuk in Mishlei seems to state outright that one should not rejoice at the downfall of his enemy. As such, we do not recite the full Hallel considering the downfall and deaths of so many Egyptians at kriyas yam suf. In explanation of why both the aforementioned explanations are necessary, R. Soloveitchik explains that there are two phenomena that can obligate us in Hallel — kiddush ha’yom and miracles. The Gemara’s reason is based on kiddush ha’yom while the Midrash’s reason is based on the miracles.
Yet, the answer of the Midrash still seems unclear. According to the logic that we should not rejoice at the downfall of our enemies, we should not say a full Hallel at any point over Pesach, yet we do on the first day(s). Why is this?
As If We Ourselves Left Egypt
A possible answer can be suggested from the Haggadah itself, wherein it states that “we must view ourselves as if we were taken out of Egypt ourselves.” Thus, in the same way that the newly freed Jews were able to sing praise to God all those years ago, we can sing a full Hallel on the first day(s) of Pesach as well, since we view ourselves as if we were just freed. While this is a nice suggestion, it doesn’t seem to hold up to scrutiny as the Jewish people sang Az Yashir on the seventh day of Pesach at kriyas yam suf — specifically when we do not say the full Hallel nowadays — not on the first day when they were actually freed. If we are obligated to view ourselves as if we ourselves left Egypt, why do we not then recite a full Hallel on the seventh day of Pesach, when we actually left?!
We can suggest the following: In truth, we really should be reciting a full Hallel the entirety of Pesach, thanking God as our Savior. We cannot do this, however, because of the posuk and principle laid down in Mishlei. Therefore, we say a full Hallel only on the first day — the day we actually left Egypt — despite the fact that it is not the actual day the Jewish people sang Az Yashir during the Exodus. The only reason we can say the full Hallel even on the first day(s) (given the posuk in Mishlei) is because we view ourselves as if we ourselves have just left Egypt. On all subsequent days, however, when we are no longer obligated to view ourselves in this way, we cannot recite a full Hallel in deference to the downfall of the Egyptians. Additionally, we note that the first day of Pesach — the actual Exodus — was not actually the Egyptians’ end. Surely, it was not a good day for them, but it was not yet their true downfall. The downfall of the Egyptians really occurred at kriyas yam suf. That is when they were completely and utterly destroyed and broken. Thus, since the real downfall of the Egyptians was on the seventh day of Pesach, a full Hallel would be inappropriate then. On the first day(s), however, when the Jews were merely freed but the Egyptians were not yet really destroyed, a full Hallel is acceptable1.
To conclude, we do not say a full Hallel on what would seem to be the more correct day due to sensitivity to the loss of others — even if they are our enemy. Therefore, we commemorate the shirah of the newly freed Jewish nation on a day that was not as bad for our enemies, and on a day when we nowadays try to feel just like we did over 3,000 years ago.
1. Taking this all in an entirely different direction, one can suggest that a shorter Hallel is actually the more intense Hallel.↩