Both Young's Literal and Green's Literal indicate that no article is included for the first five days. Only on the sixth and seventh day is the article used in the Hebrew.
This would indicate an identifying of the day in a specific way. The article defines and identifies and locates a concept in a way which is not so for a concept without an article.
'Day' is defined in the passage as a change or transition (an 'evening') a period in between and a 'dawning' into a new phase (a 'morning').
At a point where there is neither sun nor moon, this definition holds good.
On the very first day, Light is separated from Dark. They do not mingle. They do not operate together.
And it is interesting to notice that, once there is light and once there is a sun and moon, the definitions indicates that the definition of darkness (the 'night) is contained within the definition of 'day' for the evening ... (then the darkness of night) ... and the morning are the 'day'.
Separated, yet one (the dark) is bracketed within the other (the light of the morning and evening). Dark has been 'trapped' within the light, yet is not absorbed by it or mingled with it - there is separation.
Whatever occurs within it, is kept separate from the light.
Thus 'day' makes progress, an ending of one phase ('evening') then a period of darkness but despite that period of light-absence there is movement forward with another phase ... the 'morning'.
Thus the definition of day shows that darkness will not hinder the progress of the phases. It is there, but it is encapsulated within a framework of light ... fading light, then dawning light, but light nevertheless. Though light fade and though darkness has its time, yet that does not hinder the progress of the day.
Day unto day uttereth speech, and night unto night sheweth knowledge. [Psalm 19:2 KJV]
Once the first five phases are completed there is an identifying, a defining, a locating of 'day' which was not there previously. There is a finality of concept with the coming of mankind.
And then another thing altogether - a rest. A completion.
The absence, then the presence, of the article is very significant.