I had no idea it was that old! Dayenu!
If we could figure out the origin of everyone’s favorite Passover song, it would be enough.
The development of “Dayenu,” that Haggadah ditty that recaps the many miracles the Israelites experienced throughout the Book of Exodus, has puzzled scholars for generations.
The song lists 15 examples of God’s grace. As the Maharal of Prague (1512-1609) noted, they can be divided into three groups of five. The first recaps the Exodus itself (“If He had brought us out from Egypt and not carried out judgements against them … it would have been enough”). The second focuses on the miracles post-liberation (“If He had split the sea for us and had not taken us through it on dry land …”). The third group focuses on the ultimate goal of the freedom granted to the slaves — their covenant with God (“If He had given us the Shabbat and not brought us before Mt. Sinai … If He had given us the Torah and not brought us into the land of Israel … it would have been enough).
Fifteen is a symbolically important number in Judaism. The two-letter name of God, consisting of a Hebrew letter Yud and a Hey numerically adds up to 15, and there are 15 Songs of Ascent in the Book of Psalms corresponding to steps that led from one courtyard to another in the Temple.
https://jewishjournal.com/judaism/380220/dissecting-the-origins-of-dayenu/