How did Yisrael's eating of the meat of the Pesach sacrifice save them from the decree of Haman?
(12,8) "And they shall eat the meat on this night…"
The Yalkut Shimoni says that the taking of the Pesach lamb stood for them at the Jordan river, and the eating of the sacrifice stood for them in the days of Haman, as it says "And they shall eat the meat on this night", and it says in Megillas Esther 6:1 "On that night, the king's sleep was disturbed". This Midrash is a puzzle. In addition, the posuk from Megillas Esther is problematic, because it could have simply written "in the night, the king's sleep was disturbed". Why did it write specifically "that night"?
We can understand this Midrash once we understand one of the differences between Purim and Pesach. Why on Pesach did Hashem fix the festival on the day of the miracle itself, on the day that they went out, but on Purim the celebration was not fixed on the thirteenth, the day of the miracle of Yisrael's salvation, but on the day after, the fourteenth. If the reason was because on the thirteenth they had not yet rested from their enemies, as it mentions in the Megillah 9:22, they should have waited until the following year and fixed it on the day of the miracle.
We also need to understand Chazal’s teaching that on Purim "they observed and accepted" 9:27 - they observed that which they had already accepted, that is, the Torah. Until now they had observed the Torah through coercion, but now they did so willingly. But why was it that they accepted it willingly only now?
But first we need to understand why Haman fixed the decree to annihilate the Jews for the thirteenth of Adar. Because granted before the date of the death of Moshe Rabbeinu, the seventh of Adar, he was afraid to fix the decree because he was concerned that the merit of Moshe would protect them, but he could have fixed the decree from the seventh of Adar and onwards. But the reason why he did not do so was because he was concerned that perhaps the days of mourning for Moshe would protect them, like we find in the Midrash by the generation of the flood, that Hashem delayed bringing the flood until the seven days of mourning of Mesushelach were completed. So it seems to me, and I think I saw in the sefer Ya'aros Devash that he wrote the same.
But that still leaves us with a problem, because the day of the thirteenth of Adar is the seventh day of mourning for Moshe, and so Yisrael should still be protected from Haman's decree. However, since we hold that part of a day is considered like a whole day, the days of mourning were considered to be completed from the beginning of the morning onwards, and thus Yisrael would no longer have this merit to protect them.
However, the rule that part of a day is like a whole day is true only by Yisrael, because by them the day follows the night. Therefore, since there is the whole night and part of the day, most of the day has passed, and so it is as if the whole day has passed. But with non-Jews where the night follows the day, then at the beginning of the day there is only a part of a day and none at all of the night, and therefore the rule of part of the day is like the whole day cannot apply to them, because it is not sufficient to have part of the night, but rather there must be the whole night and part of the day.
According to this, we can explain that until now Yisrael had no wish to claim that they had been were forced to accept the Torah and thereby annul their acceptance of the Torah. But in the days of Mordechai they had to do this, because if their acceptance of the Torah had been done willingly, then they would have the law of Jews and so be bound by the rule that the day follows the night. Thus, on the day of the thirteenth the period of mourning for Moshe would have been completed, and so they would have nothing to protect them.
But by annulling their acceptance of the Torah they would then have the law of non-Jews and so the night follows the day, and thus they would still be protected by the days of mourning. Therefore, they annulled their acceptance of the Torah, and consequently afterwards they needed to accept the Torah again, this time willingly.
This is the meaning of what it says in Megillas Esther 9:1 “on the day that the enemies of the Jews hoped to dominate them”, that is, they hoped to dominate the Jews with them, with that which belongs to them - with the rule that the day folows the night and thus part of a day is like a whole day. "But it was reversed, that the Jews dominated their enemies", with the rule which belongs to their enemies, that the night follows the day, and so part of a day is not like a whole day.
The result of all this was that they were not able to fix the thirteenth as a day of feasting, since their salvation on that day was due to it being considered as part of the mourning period for Moshe, and thus protected them, so how could they fix it as a day of feasting and rejoicing? Therefore they fixed it on the fourteenth. And if the celebration in the walled cities was also fixed for the fourteenth, there would be no distinction in Eretz Yisrael between the walled and unwalled cities, as Chazal explain. Therefore, they fixed the fifteeenth for the walled cities.
We can now return to the Yalkut that we started with, which brought the posuk "on that night the king's sleep was disturbed". Now, this insomnia occurred on the night following the first of Esther’s feasts, and Haman’s making of the gallows, both of which occurred on the fifteenth of Nisan according to what we learn from Chazal. If so, how can the posuk say "on that night", which implies the night belonging to the fifteenth, which is the night before the feast?! But the posuk wishes to hint that their salvation was now beginning, and that Yisrael were to be judged like non-Jews and so the night follows the day. Therefore the posuk correctly says "that night", the night of the fifteenth which followed the feast of Esther.
And concerning the eating of Pesach sacrifice that the yalkut mentions, although it is slaughtered during the day, the eating is done at night, because with sacrifices the night follows the day. Now with all other sacrifices it is not so clear that the night follows the day, because maybe sacrifices are supposed to be eaten on the day that it is slaughtered and the night of the following day. But since by the Peasch sacrifice it says “and they shall eat the meat on this night”, it can only mean the night of the day of the sacrifice. If so, it is clear from here that the night follows the day.
This is what it says in the Yalkut - the eating of the Pesach sacrifice stood for them in the days of Haman, because it says “and they shall eat on this night”, which teaches that the night follows the day, and this is the rule which saved them from the decree of Haman, as alluded to by the posuk “on that night the king's sleep was disturbed”.
All this helps us to understand the posuk in Megillas Esther 9:22 “and the month which was turned for them from sorrow to joy, and from mourning to Yom Tov”. This posuk is difficult to understand, because it is talking about the month of Adar, long after the decree had been annulled, and everyone knew this. Therefore, there had never been mourning and sorrow in this month, and so it should have said something like "the month which had been fitting for sorrow". But "turned from sorrow" implies that there had been sorrow. But according to what I have written we understand that it refers to the sorrow and mourning for the death of Moshe Rabbeinu, and because of this the month was turned to one of rejoicing and Yom Tov.