Genesis 1:5

One day: According to the order [language] of the section, it should have written, "the first day," as it is written with the other days - second, third, fourth - [so] why is is written, "one?" Because the Holy One, blessed be He, was one in His world, as the angels were not created until the second day. Thus is it explained in Bereishit Rabbah 3:8.

And the Lord called the light Day; and He made it that the inhabiters of the world might labour by it: and the darkness called He night; and He made it that in it the creatures might have rest. And it was evening, and it was morning, the First Day.

“And He called” – The hey of layla is added, and it [i.e., the word] is accented on the penultimate syllable, and it is always masculine. “Evening” [erev - close in meaning to darkness, and it’s called thus since forms are mixed together [nitarvu i.e., indistinct]. “Morning” [boker] – the opposite of evening, for a person is able to check [l’vaker, i.e., to distinguish] between forms. The meaning of “one day” is referring to the rotation of the sphere. There is a secret to the midrash of six thousand. And after it has said that the light is called day, it isn’t possible that the evening should be called day, rather its explanation is “it was evening and also morning of Day One”, for if it was [referring to] the evening and morning(g of one day, what would be the meaning of “a second day”?

AND G-D CALLED THE LIGHT DAY. The verse states that time was created, and G-d made the length of the day and the length of the night.
The purport of the word vayikra (And He called) is [to indicate that] since Adam later gave names [to all the beasts, the fowl, etc.], it states that those things which were made before his existence were given names by G-d. This is the opinion of Rabbi Abraham ibn Ezra.
The correct interpretation is that the matter of calling a name here indicates the division which bounded them when they assumed their form. Thus did the Rabbis say: “[G-d said to the light,] ‘The day shall be your boundary,’ [and to darkness He said,] ‘The night shall be your boundary.’”

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.

ולחשך קרא לילה, “whereas He had already named the darkness ‘night.’” Seeing that mention of light always precedes mention of darkness, the Torah first told us of the name G’d gave to “light.”

ויהי ערב ויהי בקר. It is noteworthy that the Torah here did not write ויהי לילה, ויהי יום, but it wrote ערב. This is the period when G’d made the light of the first day fade. The subsequent expression ויהי בקר refers to the morning after the night when dawn rose. At this point the first of the 6 “days” of G’d’s creative activity of which He spoke in the Ten Commandments had come to an end. After that the second “day” began and is introduced with the words ויאמר אלוקים יהי רקיע בתוך המים. The purpose of our verse here was not to tell us that that evening and morning were part of the same day. The verse was needed only to tell us how the 6 “days” are accounted for, i.e. that the morning completed the night, which was the end of that day and the beginning of the second day.

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.

ויהי ערב ויהי בקר, although G’d had made a separation between the light and the darkness, assigning to each different time frames in which to be active, independent of the planet earth revolving on its axis, He arranged for a transition from one phenomenon to the other to take place gradually, step by step. This occurred by means of inserting a period known as evening preceding total night, and a period of dawn preceding bright sunlight, daylight.

ויקרא..ולחשך קרא לילה, the period during which there was no light was called “night.” Such a name can be applied to night even though night is something abstract, as is death, מות. We have numerous nouns which are applied to things which are abstract, such as “blindness, nudity, etc.” כסילות, foolishness, פרץ, “a breach,” and similar nouns all describe matters which are not tangible.