The kri'as haTorah for acharon shel Pesach is the parsha of the moadim found in Parshas Re'eh. In the last verse speaking about Pesach (16:8) the Torah teaches "...and on the seventh day shall be an assembly (atzeres) to Hashem, Your G-d, you shall not perform labor." Interestingly, the Torah does not provide us with a reason for this day being designated as a yom tov. Rav Hirsch understands the atzeres gathering as a gathering together of ideas and a type of recapitulation of the lessons and concepts that the yom tov provided for us. Seforno understands this day as a commemoration of the shirah - song that the Jewish nation sang upon its deliverance, hence the day was consecrated as a yom tov. Thus the very character of this day may be defined by the shirah.
The yom tov of Pesach is permeated with hakoras hatov - thanksgiving and recognition of the kindnesses that Hashem performed on our behalf. The Seder is replete with hakoras hatov; we fulfill the mitzvah of sippur yetzi'as Mitzraim by citing and explaining the four verses from the declaration of thanksgiving that the Jewish farmer recited upon bringing his first fruits to the Beis Hamikdash. Furthermore, our meal is eaten between the first two paragraphs of Hallel and its conclusion, to demonstrate that the meal itself is a seudas hodaya - a meal of thanksgiving.
Indeed, there are many lessons of hakoras hatov that emerge from the narrative in Shemos. Moshe is told not to strike neither the water to bring about the plagues of blood and frogs, nor the earth to commence kinim - lice, and thus the important lesson of gratitude, even to the inanimate, was emphasized. Even prior to the makos, when Moshe finally acquiesces and agrees to assume the mantle of leadership to be the emissary of Hashem to Pharaoh and the Jewish people, prior to going to Egypt Moshe asks permission from Yisro to leave, even though Moshe is almost eighty years old and even though it will delay his mission, because Yisro afforded him hospitality and he had to show hakoras hatov. Moreover, hakoras hatov is extended to the dogs (see Shemos 22:30) in appreciation of their having remained silent from barking on the night of makos bechoros, thereby creating a greater distinction between the crying and wailing in the Egyptian communities and the total silence in the Jewish sector.
Hakoras Hatov is such a crucial element of human decency that the Torah prohibits us from marrying an Amonite or Moavite, even if they convert to Judaism, for all time because they did not extend any help, even bread and water, to the fledgling Jewish nation upon its leaving Egypt. They should have felt a debt of gratitude to us since our grandfather Avrohom saved their grandfather Lot, and yet they did not return the kindness (Devorim 23:5) and thus are forever banned from marrying into the Jewish people. Rav Chaim Friedlander zt"l notes (Sifsei Chayim Volume 2, Page 278) that the Jewish people were being nourished miraculously by the mon and the be'er of Miriam, and therefore we did not actually need any help from Amon and Moav; rather it was they who needed to extend basic goodness and kindness in order to show appreciation, but failed to do so. Hashem, who is not only our Liberator but our loving Father and Teacher, says regarding yetzi'as Mitzrayim, "Today you are leaving in the month of spring" (Shemos 13:4). Rashi explains that Hashem was not telling them the time, rather He was noting that they should see the kindness that He bestowed upon them by taking them out during the most comfortable season of the year.
I believe that shevi'i shel Pesach, complete with the singing of shirah, brings us to a newer, higher, pinnacle of hakoras hatov. At first glance the essence of the Shirah is thanking Hashem for saving us from the danger of the Egyptians who were seeking to either kill us or return us to Egypt for further bondage. However, the Beis Halevi teaches that core idea of the shirah is found in its very last passuk (15:19), "When Pharoh's horse came with his chariots and horsemen into the sea and Hashem turned back the waters of the sea upon them, the children of Israel walked in the dry land amid the sea." The prior eighteen verses are all extolling Hashem as the incredible warrior who, with such precision, is able to simultaneously punish each Egyptian in accordance with the level of punishment that they deserved (See Rashi 15:5). The hakoras hatov of the shirah is understood by the Beis Halevi as one of thanksgiving to Hashem for using Bnei Yisrael as the vehicle through whom a magnificent kiddush Hashem occurred. Hashem changed the natural order, transforming water into dry land on behalf of Bnei Yisrael and the entire world, without the benefit of modern communications, knew of the miraculous splitting of the Red Sea. Rachav (Yehoshua 2:10) tells the spies Yehoshua sent that, "we have heard how Hashem dried up the water of the Sea of Reeds." This is even a higher and more profound form of hakoras hatov; it is not only thanksgiving for a physical deliverance but for spiritually elevating former slaves by allowing them to participate in bringing about honor and splendor to the King. Yetzi'as Mitzrayim connotes not only our transformation from slaves to free men, but from slaves to aristocracy. I believe this very special designation came about at Kri'as Yam Suf.
The Seforno explains the possuk, "...that is this night unto Hashem guardings for all the children of Israel for their generations" (Shemos 12:42) to indicate that just as Hashem anticipated anxiously the night of redemption in Egypt, so too, He anxiously yearns for our redemption in the future. Moreover, the Navi (Micha 7:15) prophesies that just as Hashem showed wonders to the Jewish nation upon our deliverance from Egypt, so will He in the future. Our prayer as we enter the last days of this holiday is for Hashem to once again redeem us and use us, His children, as the vehicle of educating the entire world about His existence and His magnificence.