I saw a nice pshat this past shabbos.
Rashi says that the reason why by the first day of creation it says “Va’yehi Erev Va’yehi Boker Yom Echad” and not Yom Rishon, as the sequence from the other days of the week goes (yom sheni, shlishi, etc), is because since Hashem created spiritual beings (malachim) on the second day, He was therefore the only living Being around, it was Yom Echad, the day of the One.
It always bothered me, why was this so important to say? and if he wasn’t, if malachim were created on that first day. then what? then he’s NOT echad (c”v)? What lesson, yesod, principle do we glean from this that it was “the day of the One”??
So I actually saw in a sefer called Pri Dovid an answer to this question. That it is to tell us something extremely fundamental to our yiddishkeit and hashkofo. He cites different seforim who tell us that He created this world out of Goodness, and that it was created solely for the benefit of Man. Hashem Himself has zero benefit per se and was totally unaffected with His Handiwork. The same Hashem as we know Him now, was the same Hashem before the world came into existance. He is totally out of the realm of space and time and everything else we have in this universe.
This perhaps is what the torah was teaching us on Yom Echad. He was the same one Hashem,even after the world came into being, without any change, benefit, or affect. It was “rak v’ach” a creation for the sole benefit of mankind.