Exode 12:42

Four nights are there written in the Book of Memorials before the Lord of the world. Night the first,--when He was revealed in creating the world; the second,--when He was revealed to Abraham; the third,--when He was revealed in Mizraim, His hand killing all the firstborn of Mizraim, and His right hand saving the firstborn of Israel; the fourth,--when He will yet be revealed to liberate the people of the house of Israel from among the nations. And all these are called Nights to be observed; for so explained Mosheh, and said thereof, It is to be observed on account of the liberation which is from the Lord, to lead forth the people of the sons of Israel from the land of Mizraim. This is that Night of preservation from the destroying angel for all the sons of Israel who were in Mizraim, and of redemption of their generations from their captivity.

It was a night of watching unto the Lord: According to its simple understanding, since it mentioned, "it was a night of watching unto the Lord," and at the end, "watching... for all the Children of Israel throughout their generations" - it appears that the explanation is that because the Lord watched them and did not let the destroyer come to their homes to attack, He commanded that this night be preserved by all of the Children of Israel throughout their generation. And [this is by way of] eating the Passover sacrifice, according to its statute, and matza and bitter herbs on that night. But some understand it's explanation to be like the explanation of "the watchers of the [city] walls" (Song of Songs 5:7), such that they not sleep, but rather give thanks and recount the powerful acts of the Lord upon their leaving Egypt. And the Sages, may their memory be blessed, hint to this - "the time of reading Shema in the morning has arrived" (Passover Haggadah).

IT WAS A NIGHT OF WATCHING UNTO THE ETERNAL FOR BRINGING THEM OUT FROM THE LAND OF EGYPT. The verse is stating that from the time He decreed the exile upon them, He observed the matter that He bring them out on that night once the end had come, for I will hasten it in its time. It may be that the verse, It was a night of watching unto the Eternal, means that He was watching and looking forward to the night when He would bring them out from the land of Egypt, for the Holy One, blessed be He, looked forward to the time when they would merit to be brought out therefrom.
Now if we are to say [as Rashi did, quoted in the commentary on Verse 40], that the [period referred to in] the verse, that thy seed shall be a stranger in a land that is not theirs,. begins from the time that Abraham had seed, and that the reckoning [of the four hundred and thirty years] begins from the birth of Isaac, you will find that they stayed in Egypt two hundred and forty years, according to the explanation we mentioned., 47:9). We thus have one hundred and ninety years since the birth of Isaac. Deduct them from the sum of four hundred and thirty, and you have two hundred and forty years remaining for the stay in Egypt. But this too in my opinion is not correct according to the plain meaning of Scripture, since all the days of Abraham cannot be counted as exile with respect to his seed.
The correct interpretation is that He was saying to Abraham “that thy seed shall be a stranger in a land that is not theirs … four hundred years. from this day on.” The purport thereof was to tell him: “your children will not immediately inherit this land which I give them, but instead they will be strangers like you were, in a land not theirs [for a period of] four hundred years and more. They will not return here till the fourth generation when four hundred and thirty years will be completed.” But if so, then their stay in Egypt lasted about two hundred and twenty years or thereabouts. Now if the numerical value of the word ‘r’du’ (get you down) thither, (go you), but r’du (get you down), because the numerical value of the word r’du is two hundred and ten. There was thus an allusion here to the time the Israelites would stay in Egypt. [which is two hundred and ten], be an established tradition in Israel, it is possible that [Jacob, by using the word r’du], alluded to those who arrived in Egypt that after …

ליל שימורים הוא לה', G’d had been anticipating this night ever since the days of the patriarchs in order to have the opportunity to lead the Jewish people out of Egypt. For the Jewish people, on the other hand, this night now became a night to be remembered

לרורותם, for all their future generations. The expression

שמורים, represents an element of “waiting for something.” It is used in this sense in Genesis 37,11 ואביו שמר את הדבר, “his father waited to see what would happen in this matter.”

ליל שמורים הוא לה' להוציאם, the night G’d had been looking forward to, to take the Jews out from there. G’d had not caused the Israelites all this suffering in Egypt out of caprice, just in order to keep to a timetable of His, but He simply had not found them ready and worthy of redemption until that particular night. He had reserved this night for that event because He is so full of loving kindness. This is what our sages meant when they stated that G’d was מחשב את הקץ, He had manipulated history in order to bring about the redemption so much earlier than even the Israelites had thought it would come, based on the prophecy to Avraham. (Haggadah shel Pessach.)

הוא הלילה הזה לה' שמורים, just as G’d had looked for ways and means to bring the redemption to the Jewish people from their suffering in Egypt, so He is looking for legal ways and means to redeem us from the present exile and to bring on the final redemption. Isaiah phrased this asולכן יחכה ה' לחננכם, “Truly, the Lord is waiting to show you grace.” (Isaiah 3018)

לכל בני ישראל לדורותיכם, in accordance with the statement of our sages in Rosh Hashanah 11 “just as the Israelites have been redeemed from Egypt in the month of Nissan, so the final redemption will also occur during the month of Nissan.”