As children, we were all taught that we celebrate Shevii Shel Pesach (the 7th day of Pesach) because on the 21st of Nissan, Hashem split the Yam Suf, saving Bnei Yisrael and killing the Egyptians. But this cannot possibly be the reason, since we were commanded to observe Shevii Shel Pesach even before Hashem split the sea! The Pasuk in Parashat Bo, right after introducing Makat Bechorot, states, “UVaYom HaRishon Mikra Kodesh, UVaYom HaShevii Mikra Kodesh Yihyeh Lachem,” “And the first day shall be a holiday, and the seventh day shall be a holiday for you” (Shemot 12:16). Thus, it is evident that the basis for the celebration of both the first and seventh days of Pesach relates to Hashem saving Bnei Yisrael from Makat Bechorot and taking them out of Egypt. However, it is strange that the celebration of the 21st day of Nissan is not mentioned in Parashat BeShalach and that it is never connected to the miracle at Yam Suf that took place on that day.
The Meshech Chochmah explains that if Hashem had commanded us to observe the 21st day of Nissan as a holiday after He split the sea, it would appear as if Bnei Yisrael were celebrating the downfall of their enemies. This is not, however, why Bnei Yisrael celebrate on this date. This is demonstrated by the fact that we do not recite full Hallel on Shevii Shel Pesach. In fact, the nature of many other holidays throughout the year shows that we do not celebrate the downfall of our enemies. On Chanukah, we celebrate the miracle of the oil and do not celebrate the fact that we destroyed our enemies. (This is the reason mentioned in the Gemara. In fact, the Al HaNissim prayer does refer to the miracle of the victory. See Rabbi Jachter’s article available at www.koltorah.org regarding the debate over reciting Hallel on Yom Haatzmaut, which discusses this point.) Purim is celebrated on the 14th of Adar, which is not when Haman was hanged or when Bnei Yisrael killed their enemies; rather, Purim is celebrated because we rested on the 14th. Similarly, Hashem commanded us to observe Shevii Shel Pesach before the miracle at Yam Suf to demonstrate that we are not celebrating the downfall of our enemies.
If we do not celebrate the defeat of the Egyptians on Shevii Shel Pesach, then why did Bnei Yisrael sing Shirah immediately after crossing the sea? Rabbi Tuvia Grossman suggests that even after Bnei Yisrael were physically freed from Egypt, they remained terrified of the Egyptian empire that had enslaved them for so many years. It was only after the miracle at Yam Suf that Bnei Yisrael realized they had not only escaped the people of Egypt – the Mitzrim – but also the empire and culture of Mitzrayim, as the Pasuk states, “VaYar Yisrael Et HaYad HaGedolah Asheir Asah Hashem BeMitzrayim,” “And Bnei Yisrael saw the ‘strong hand’ which Hashem had used against Mitzrayim” (Shemot 14:31), – not BeMitzrim (against Egypt and not against Egyptians). At this point, they sang Shirah.