Genesis 1:5

יום אחד THE FIRST DAY (literally, one day) — According to the regular mode of expression used in this chapter it should be written here “first day”, just as it is written with regard to the other days “the second”, “the third”, “the fourth”. Why, then, does it write אחד “one”? Because the Holy One, blessed be He, was then the Only One (Sole Being) in His Universe, since the angels were not created until the second day. Thus it is explained in Genesis Rabbah (Genesis Rabbah 3:8).

וַיִּקְרָ֨א אֱלֹהִ֤ים ׀ לָאוֹר֙ י֔וֹם וְלַחֹ֖שֶׁךְ קָ֣רָא לָ֑יְלָה וַֽיְהִי־עֶ֥רֶב וַֽיְהִי־בֹ֖קֶר י֥וֹם אֶחָֽד׃ {פ}

And God called the light Day, and the darkness He called Night. And there was evening and there was morning, one day.