Midras Mazmur 94:1: Tanna DeBei Eliyahu Rabbah, Ein Yaakov (Glick Edition), Shir HaShirim Rabbah & Midrash Tehillim

Alternatively, "These days were formed, but not one from them" [Psalms 139:16] is refering to Yom Kippur for (the nation of) Israel. {continuing}

Fol. 31) We are taught in a Baraitha: R. Juda said, in the name of R. Akiba: "What [song] did [the Levites] chant on the first day of the week? (Ps. 24) The earth is the Lord's and the fulness thereof, because He [the Lord, in establishing the world] took possession and gave [His creatures] possession (invested them with life), and [became] the Sovereign of the world. What did they sing on the second day? (Ib. 48) Great is the Lord and greatly to be praised, because [on that day] He divided His works [the heavens from the earth] and reigned over them. On the third day they sang (Ib. 82) God standeth in the congregation of the mighty, because [on that day] He, in His wisdom, caused the earth to be revealed and the world to be prepared for its occupants. On the fourth day they sang (Ib. 94) O Lord, to whom retribution belongeth, because [on that day] He created the sun and moon, and in the future He will punish those who would worship them. On the fifth day they sang (Ib. 81) Sing aloud unto God, our strength, because [on that day] He created the birds and the fishes [which bring] praise to His name. On the sixth day they sang, (Ib. 93) The Lord reigneth. He is clothed in majesty, because [on that day] He finished His works and became sovereign over them. On the seventh day they sang (Ib. 92) A psalm, a song, for the Sabbath day; i.e., for the day that is a perfect rest." R. Nechemia said: "What reason had R. Akiba to make distinctions among these sections, [for the last refers to a future event, while all the others refer to the past]? We must therefore say that they sang that psalm on the Sabbath day, because He rested." This is contrary to the opinion of R. K'tina, for R. K'tina said: "The world will last six thousand years, and one thousand years it will be waste; as it is said (Is. 2, 11) And exalted shall be the Lord alone on that day" Abaye, however, said: "Two thousand years it will waste; as it is said (Hos. 6, 2) He will revive us after two days."

Another matter: “Your hair is like a flock of goats that streams down [shegaleshu] from Mount Gilad” (Song of Songs 4:1) – the mountain from whose midst you took away [shegelashten], I rendered a memorial for the nations of the world. Which is this? These are the priestly watches. What is it that you took away from its midst? “Your teeth are like a flock of ordered ewes” (Song of Songs 4:2) – defined matters, the twenty-four priestly watches, the twenty-four Levite watches, and twelve divisions.⁵⁴These were divisions of men who would come for a month at a time to perform whatever labors the king would assign them (Etz Yosef). “That have come up from bathing” (Song of Songs 4:2) – who protect Israel. “That are all paired” – as we learned there: At three times during the year all the priestly watches were equal (Sukka 55b). “And there is none missing among them” (Song of Songs 4:2) – as we learned: “The first, the head and the hind leg…” (Tamid 31a).⁵⁵Each of the priests had a role, and there were always enough priests to perform each task.
“Your lips are like a scarlet thread” (Song of Songs 4:2) – as we learned: [The High Priest] stooped to pour the libation, and the deputy waved the cloths, and ben Arza struck the cymbals (Tamid 33b).⁵⁶This was the sign for the Levites to begin their song in the Temple. “Your speech is lovely” (Song of Songs 4:2) – this is the song, as we learned there: The song that the Levites would recite in the Temple. On the first day they would recite: “The earth is the Lord’s and all it contains” (Psalms 24:1).⁵⁷The intention is that on Sunday the Levites would recite the entire chapter of Psalms beginning with this verse. The same is true for all of the citations cited here; one psalm was recited each day of the week. On the second day: “Great is the Lord and highly to be praised in the city of God…” (Psalms 48:2). On the third day: “God stands in the divine congregation” (Psalms 82:1). On the fourth day: “Lord God, to whom vengeance belongs, God to whom vengeance belongs, shine forth” (Psalms 94:1). On the fifth day: “Sing for joy to God, our strength; shout aloud to the God of Jacob” (Psalms 81:2). On the sixth day: “The Lord reigns: He is robed in majesty” (Psalms 93:1). On Shabbat they would recite: “A psalm, a song for the Shabbat day” (Psalms 92:1) – for the future, for the day that will be entirely Shabbat and rest for everlasting life.