Commentary for 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).

ויקרא אלוקים ליום אור, you may ask yourself what possible need there was for G’d to call this light “day” already at the time He created it; however this is the way Moses worded it when he wrote down the Torah. Whenever, in Scriptures, G’d is recorded as mentioning “day” and “night,” as for instance in Genesis 8,22, where He made the promise that “henceforth the alternating manifestations of day and night will not cease to occur regularly,” יום ולילה לא ישבותון, this is a reference to the original light created on the first day. G’d always called this “light” “night and day.” Similarly, every time in this chapter when we encounter the expression ויקרא אלוקים, as well as when we read in Numbers 13,15 ויקרא אלוקים להושע בן נון יהושע, whereas the same man as representative of the tribe of Ephrayim had been referred to as הושע only 7 verses earlier as הושע. This was the same man, who, previously, when he appeared in his position of Moses’ personal valet was already known as יהושע, (Exodus 24,13) It is customary among kings that when they appoint a person to a position of eminence to signal this by changing the name of such a person. One well known example of this is Pharaoh calling Joseph צפנת פנח upon appointing him as viceroy of Egypt (Genesis 41,45) Other such examples are found in Daniel 1,7, etc.

ויקרא אלוקים לאור יום; even though at that stage of creation “time” was not yet an operative term as we know it nowadays, i.e. the terms “day” and “night” were not yet used by anyone, G’d named these phenomena as such already at that time.