Bereshit Rabbah 3:8 (Vilna numbering; Theodor–Albeck differs)
“It was evening” (Genesis 1:5) – these are the actions of the wicked. “And it was morning” (Genesis 1:5) – these are the actions of the righteous. “[It was evening and it was morning,] one day” – the Holy One blessed be He gave to them [to Israel] one [unique] day. Which is that? It is Yom Kippur. Rabbi Tanḥum bar Yirmeya said: [It was the day] on which four items were created: Mountains, the heavens, the earth, and light. Rabbi Yudan said: [It was the day] on which the Holy One blessed be He was alone in His world, as there was no one else in His world other than Him. This is in accordance with the opinion of Rabbi Yoḥanan, but it is not in accordance with the opinion of Rabbi Ḥanina. Rabbi Yoḥanan said: The angels [malakhim] were created on the second day. That is what is written: “He covers His upper chambers with water; He makes clouds His chariot; He proceeds on the wings of wind” (Psalms 104:3), and it is written: “He makes the winds His messengers [malakhav]” (Psalms 104:4). Rabbi Ḥanina said: The angels were created on the fifth day, as it is stated: “Let birds fly [yeofef] over the earth…” (Genesis 1:20) and it is written: “And with two it would fly [yeofef]” (Isaiah 6:2). Rabbi Lulyana bar Tavrai said in the name of Rabbi Yitzḥak: Both according to the opinion of Rabbi Ḥanina and according to the opinion of Rabbi Yoḥanan, everyone agrees that none were created on the first day, so people should not say: Mikhael was pulling at the south side of the heavens and Gabriel at its north side, while the Holy One blessed be straightened it out it in the middle. Rather, “I am the Lord, who made everything; who stretched out the heavens alone, who spread the earth by Myself [me’iti]” (Isaiah 44:24) – it [the last word] is written [as two]: “who is with Me [mi iti]?” Who was partner with Me in creation of the world?
The Sefaria Midrash Rabbah, 2022 · sefaria-midrash-rabbah
Bereshit Rabbah 3:8 (Vilna numbering; Theodor–Albeck differs) · cc-by-sa
Three answers to 'one day' stacked in one section, and the last of them is the one Rashi will carry into Christendom's reading list. (1) Evening and morning are the deeds of the wicked and of the righteous — the moral reading. (2) 'One day' is the day God gave Israel: the Day of Atonement. (3) R. Yudan: it was the day on which the Holy One was alone (yachid) in his world. That third answer forces the angel question, and the section closes by ruling that on the first day nothing whatever was created besides God — lest anyone say Michael held the sky at the south and Gabriel at the north while God measured in the middle.