Another approach to the question of why the Torah established the seventh day of Pesah as a full-fledged holiday is expressed in many traditional sources and is widely known; namely, the seventh day is, according to the Midrashim, the day on which Hashem split the Sea of Reeds, allowing the Jewish people to pass through to safety and drowning their Egyptian pursuers. This view of the origin of the special status of the seventh day of Pesah is reflected in the Torah reading for that day - we read the beginning of Parashat Beshalah, which describes the Jews' exit from Egypt, Pharaoh's final change of heart, and the ensuing drama at the Sea.
The first difficulty with this explanation is a basic one. The Exodus was the culmination of a multi-stage process. It did not occur in one fell swoop. Yet we do not find that the Torah requires us to commemorate each of the ten plagues with a distinct holiday; we reflect upon all of them, and the lessons they represent, on the first day of Pesah.
On the surface, it would seem that the splitting of the sea is no different. It was certainly a magnificent and miraculous occurrence that is worthy of mention at the Seder; however, all things considered, it is just another component of the process by which the Jews were led by Hashem to freedom. Why does the splitting of the sea justify the creation of another full fledged holiday any more than any of the other plagues that befell the Egyptians and facilitated our liberation?
A second, equally significant difficulty pertains to the story of the splitting of the sea itself. Pharaoh had acquiesced, albeit after much resistance, to Hashem's command to release the Jewish people from bondage. They were well on their way to receive the Torah and settle in the Land of Israel. Apparently, though, Hashem was not satisfied with this outcome. He insisted upon enticing Pharoah to pursue his former slaves, so that he and his army would meet their demise in the Sea of Reeds. Why wasn't the liberation of the Jewish slaves sufficient? Why was it necessary, from Hashem's perspective, to punish the Egyptians further?
I would suggest the following answer: Through their experience of the plagues and their witnessing the remarkable turn of events in Egypt, the Jewish people had become fully convinced that it was worthwhile to commit themselves to following the path of Hashem and to leave Egyptian life behind once and for all. This, in and of itself, was wonderful, but was not a complete 'redemption'.
If Pharaoh had resumed business as usual after the departure of the Jews, he may eventually have seen a rehabilitation of the Egyptian economy and infrastructure, etc. The ordeal with the plagues would have registered in the minds of the Egyptians and the Jews, in retrospect, as a challenge to Pharaoh's sovereignty that was ultimately overcome. Pharaoh took a beating but was not defeated; he may have lost the battle but he was fully capable of continuing the war if need be. While life under God's Kingship might have been proven to be viable, life under the kingship of Man would still exist, at least in theory, as a reasonable (and tempting) alternative. Torah and Egyptian culture would have been seen as two equally valid options - each with their positives and negatives, their strengths and weaknesses - but two options of substance nonetheless.
For this reason, Hashem orchestrated the splitting of the Sea and saw to it that the Children of Israel witnessed the drowning of their oppressors in its depths, as the Torah states:
And Israel saw the Egyptians dead on the bank of the Sea.
The Talmud comments that, before they saw the corpses of the Egyptians wash ashore, the Jews imagined that their tormentors had survived on the other side of the Sea, that they had withdrawn from the water before it was too late. In other words, they fantasized that the kingdom of man could prevail; although it might lose some battles, it remained a contender in the cosmic struggle for mastery of the Universe. Hashem, therefore, willed that the dead Egyptians be seen by the Jews, so that the harsh reality of their fate would be known beyond the shadow of a doubt.
And it is precisely because the Jewish people were then totally disabused of any fantasies about the potential of human dominion to contend with God that we read:
And Israel saw the great hand that Hashem manifested against Egypt, and the people feared Hashem; and they trusted Hashem and Moshe, His servant.
Indeed, the conclusion of the Song at the Sea summarizes the lesson of this dramatic event in one sentence:
Hashem will reign for all eternity -
That is, He did not merely prevail in a single battle, only to be challenged by Pharaoh or his ilk again in the future; on the contrary, He is now seen to transcend and direct any and all material forces, human or otherwise, and His kingship is therefore acknowledged as absolute.
We now understand why, whenever the Exodus is mentioned in our liturgy, the downfall of Pharaoh is emphasized alongside the salvation of Israel. There are numerous instances of this, but one that immediately comes to mind is in the blessing that follows the Shema each morning:
From Egypt did You redeem us, Hashem, our God; from the house of slaves you freed us. All of their firstborn did You kill, and Your firstborn, Israel, did You redeem, and You split the sea for them - the wicked You drowned, and the beloved ones passed through the sea; water covered their oppressors, not one of them was left.
It is not enough, then, to recognize the greatness of Hashem and His majesty alongside that of man; if our redemption is to be complete, we must become fully cognizant of the futility of human dominion in the Universe, and utterly reject the validity of any alternative to the service of our Creator. We do not simply choose the worship of Hashem as a good among other possible goods; on the contrary, we consider all other purported "options" illusory.
This attitude is captured beautifully in the closing verse of the first chapter of Tehillim:
For Hashem knows the way of the righteous, and the way of the wicked will be destroyed.
This, then, is the importance of the seventh day of Pesah and the splitting of the sea that occurred on it. The plagues that led to the Exodus, and the commandments the Jews were required to perform before their release from bondage, were all designed to provide the Jewish people with the education they needed to be ready to respond to Hashem's directives. The plagues convinced them of the reality of Hashem's existence and the overwhelming power of His governance, and their participation in the Paschal sacrifice demonstrated their commitment to His service. During the first six days of Pesah, we explore and attempt to internalize these themes.
The splitting of the sea, on the other hand, was not intended to establish the basic reality of Hashem and His providence but to show that there is no other reasonable basis for a meaningful and fulfilling life - whatever seems to offer such a basis is, ultimately, a house of cards waiting to be toppled. Commemorating the splitting of the sea on the Seventh Day of Pesah completes our acknowledgement of God's sovereignty by reminding us that "the way of the wicked will be destroyed"; it underscores not only our acceptance of Hashem's kingship (already manifest on the first six days of Pesah) but our absolute rejection of any humanly crafted alternative.
Thus, it is only on the Seventh Day of Pesah that we declare, once and for all, that "Hashem will reign for all eternity."
(Incidentally, we can also see from this analysis why the Seventh Day is not a completely independent holiday - it is still "Pesah", does not require a separate sheheyanu blessing, etc., because it is meant to further develop and round out our reflection upon the theme of Hashem's sovereignty introduced on the First Day of Pesah. It doesn't add any fundamentally new content to the Yom Tov.)