אשירה לה' כי גאה גאה
Says the Medrash:
“Then Moshe chose to sing,” as the passuk states ”Your throne was established from of old.” Said Rav Berechya in the name of Rav Avahu, although You [G-d] have always existed, Your throne was not established, and You were not known in Your world until Your children sang this song. Therefore it says “Your throne was established…”
Alternatively, “Then Moshe chose to sing,” as the passuk states “She opens her mouth with wisdom, and the teaching of kindness is on her tongue.” From the day that G-d created the world up until Bnei Yisrael stood at the sea we do not find anyone who sang shira to G-d, except for Bnei Yisrael. He created Adam, and he did not sing shira. He saved Avraham from the furnace and from the kings and he did not sing shira. He saved Yitzchak from the knife and he did not sing shira. He saved Yaakov from the angel, from Esav and from the people of Shechem, and he did not sing shira. When Bnei Yisrael came to the sea and it split before them, at once they sang shira to G-d as it states “Then Moshe and the Children of Israel chose to sing this song.” This [is the meaning of the passuk] “She opens her mouth with wisdom.” Said the Holy One, Blessed is He, for these [people] I have been waiting…
What is so significant about this shira, this song? Why was it so important, that G-d had waited with anticipation for Bnei Yisrael to sing it?
The commentators have struggled to explain Rashi’s words on this passuk. “Then” when Moshe saw the miracle, the idea occurred to him [in his heart] to sing shira. The question is, isn’t any action preceded by an intention of the heart? If a person has no desire to do something, he won’t do it. So what, then, does Rashi mean by עלה בלבו שישיר שירה, the idea occurred to him [in his heart] to sing shira?
The Maharal asks this question:
“Then” when Moshe saw the miracle. Rashi was bothered by the letter יוד from the word ישיר which makes the word into future tense. He therefore explained that it means, “when he saw the miracle, the idea occurred to him [in his heart] to sing” which he didn’t do yet, rather it’s in the future, which explains why it is written in the future tense. Though it is still difficult why it is written that the idea occurred to him to sing when it’s clear from the psukim that he was singing already. It should have simply said, אז שר משה, “Then Moshe sang” instead of the lengthy “the thought occurred to him to sing, and he did so,” no?
The Maharal answers,
The action of song is in [one’s] heart. When there is happiness in the heart of Tzaddikim, song swells up in their hearts. Have no doubt that in every heart there was song and rejoicing. Therefore it says אז ישיר, “Then they will sing,” the idea occurred to them to sing, meaning that the happiness of the miracle entered their hearts, and they did not need to force themselves intellectually to [sing that] song like a person who forces himself to things against his will, for if that was the case, the singing would not have been with happiness. A happy song starts with a joy of the heart which leads to the idea to sing. That’s why the passuk speaks about song in the future tense.
Shira is not just a thank you. It is not merely recognizing the miracle and singing praise. Shira is an outburst of the soul, where the soul is elevated due to happiness. It is not possible to sing a shira without the heart swelling with joy. When a person takes to heart all the miracles Hashem does for him with love, when he feels close to Him, his heart swells with happiness and the shira bursts out in praise of G-d. As Rashi points out אין אדם שר שירה אלא מתוך שמחה וטוב לבב a person only sings shira from happiness and good heartedness as the verse says, עבדי ירונו מטוב לב My servants will exalt from good heartedness.
What is the nature of this joy, this precursor to shira? Shira is a song that is sung upon witnessing completeness, upon viewing the whole picture. When a person witnesses a miracle, when he examines how every minute detail was divinely orchestrated by G-d to bring about that miracle, it is then that he witnesses the miracle in all its magnitude. When Bnei Yisrael came to the Red Sea, they despaired; the sea was in front of them and the Egyptians were behind them, leaving them no place to go. When they saw this, they wondered when the promised redemption would be completed, when will they see in hindsight that all their hardships and suffering were stepping stones to freedom. The Children of Israel raised their eyes and behold!—Egypt was journeying after them, and they were very frightened; the Children of Israel cried out to Hashem. Once they emerged from the sea and saw the Egyptians dead on the shore, the redemption was complete and the darkness of their exile dissipated. They saw that their suffering in Egypt, the hardening of Pharaoh’s heart, was for the purpose of taking revenge from him and his people. They knew that they will never again see Egypt or the Egyptians, and it was then that they wished to sing shira.
Shira is sung only at completion. Similarly, we find when Yaakov fought with and defeated the archangel of Esav, the archangel said to him שלחני כי עלה השחר, let me go, for dawn has broken. Rashi there explains that Esav’s archangel wished to sing shira by day. The Magid of Koshnitz explained that precisely when Yaakov defeated the angel and thereby lay the foundation for all future generations of Israel that they will always defeat Esav, that is the moment when he wished to sing
shira—for shira is only sung at a moment of completion, of perfection.
From the day that G-d created the world up until Bnei Yisrael stood at the sea we do not find anyone who sang shira to G-d, except for Bnei Yisrael. He created Adam, and he did not sing shira. He saved Avraham from the furnace and from the kings and he did not sing shira. He saved Yitzchak from the knife and he did not sing shira. He saved Yaakov from the angel, from Esav and from the people of Shechem, and he did not sing shira. When Bnei Yisrael came to the sea and it split before them, at once they sang shira to G-d.
Midrash Rabbah, Shemos, 23:4
At the time of his creation, Adam HaRishon had not yet reached the completion of his destiny. He was moved by creation, and praised G-d, but never sang shira. When Avraham Avinu was saved from the furnace and from the war he fought against the kings to save Lot, the building of the Jewish nation had not yet been completed. Avraham was thankful and tithed from everything he received, but did not sing shira. Yitzchak on his return from the Akeidah, and Yaakov when he was saved from his enemies, even though these events left an indelible mark on the history of the Jews—so much so that even Esav’s archangel wished to sing shira for completing his role, namely that of being defeated—the Jews had not yet attained their ultimate completion. Yitzchak and Yaakov praised and thanked G-d, but they did not sing shira. But when Bnei Yisrael came out of the sea, their redemption was complete and having become G-d’s nation, they desired to sing shira. As the Medrash states, when You stood at the sea and we sang shira to You, it was then that your kingship and throne became established, as it says “Your throne was established from of old.”
The Ran asks why we say Hallel on Pesach night, if Hallel is only supposed to be said by day. Based on what we’ve learned, his answer is clear:
Rav Hai Gaon [said] that we do not make a blessing when saying Hallel Pesach night because we are not saying it as reading the Hallel but rather we are saying it as a shira, as it has been taught, R’ Gamliel says… therefore we have to thank and praise.
We do not say Hallel at night, only by day. The reading of Hallel is only by day. But the Hallel that we say Pesach night is not an ordinary Hallel, but a shira. One does not calculate when to sing shira—it is a bursting forth of gratitude to G-d and Pesach night this gratitude compels us to sing shira— "השיר יהיה לכם כליל התקדש חג", the song will be yours like the night of the festival’s consecration—like the song sung by those who were redeemed from Egypt.
 מדרש רבה שמות פרשה כ"ג פסקה א'
 תהלים צ"ג:ב'
 משלי ל"א:כ"ו
 גור אריה, שמות ט"ו:א
 ערכין י"א ע"א
 ישעיה ס"ה:י"ד
 עבודת ישראל פר' וישלח
 שמות רבה כ"ג: א'
 תהלים צ"ג: ב'
 פסחים פרק י'
 ישעיה ל': כ"ט