Word: יום אחד
Representation: YVM AKhD
Transliteration: Yom Echad
Noun: "one day" or "day of one"
Strong Concordance: H3117 and H259
Exposition
This is another one of those two-word expressions that needs to be explained together.
This expression appears 10 times in Scripture, and normally it just means "one day", as in Genesis 27:45, "Shall I be deprived of both of you in one day?"
Pretty straight-forward, right?
But there is one instance where it is not so straight forward: Genesis 1:5.
In all of the other naming of the days you have "second day, third day, fourth day, etc.". But the first day is not called "first day", but "one day".
It seems out of place, and there has been a lot of interpretation added to try to make sense of it. Such as "call it the 'day of One' as in speaking of the One Creator!" The problem with that is that the Elohim are a plurality, and so it is a monotheistic interpretation being forced upon a henotheistic text.
More than likely, it is just a stylistic expression since nowhere in the Tanach can you find "the first day". It just wasn't done. Why? I don't know. But I do know that because of the lack of use, the expression "one day" in Genesis 1:5 could also be translated as "a first day" as KJV nearly does (it uses "The first day", which isn't such a good choice.
But to show the uniqueness of this term, I will leave it simply as "one day" as the author intended, just to show something rather interesting.