(4) Perhaps Nahum Sarna is on the right track to suggest that the definiteness of the construct phrase for days six and seven might be due to emphasize these culminating days of creation. I also suggest that perhaps style could explain the choice of the definite construct phrase rather than the definite adjectival phrase. Notice that seven of the eleven adjectiveיום ה verses cited above are considered to be from P: Gen 1:31; 2:3; Ex 12:15, 18; Lev 14:57; 19:6; 22:27. (Ex 20:20 // Deut 5:14 might also be from a priestly editorial hand.) And one of the four adjectiveה nounה verses is considered to be from P: Num 6:12. Thus at least for the word yom there could be a preference in P for the adjectiveיום ה construction where a definite preposition is not used. Just a thought.

(4) Perhaps Nahum Sarna is on the right track to suggest that the definiteness of the construct phrase for days six and seven might be due to emphasize these culminating days of creation. I also suggest that perhaps style could explain the choice of the definite construct phrase rather than the definite adjectival phrase. Notice that seven of the eleven adjectiveיום ה verses cited above are considered to be from P: Gen 1:31; 2:3; Ex 12:15, 18; Lev 14:57; 19:6; 22:27. (Ex 20:20 // Deut 5:14 might also be from a priestly editorial hand.) And one of the four adjectiveהיום ה verses is considered to be from P: Num 6:12. Thus at least for the word yom there could potentially be some preference in P for the adjectiveיום ה construction where a definite preposition is not used. Perhaps there are certain contextual factors that play a role in this choice. It might be interesting to research adjectiveה noun and adjectiveה nounה phrases further to see if there might be a pattern in their usage. It is also possible that depending on the context there is not much difference to the author and it just so happens that one is chosen over the other. Perhaps another factor to consider is diachrony. Just some thoughts.

(1) יוֹם הַשִּׁשִּׁי (yom hash-shish-shi) is a construct phrase where the adjective שִׁשִּׁי acts as a noun. The phrase literally means "the day of the sixth" or "the sixth's day" but can simply be understood as "the sixth day". This kind of phrase isn't ungrammatical, but it is relatively rare. Here are the only twelve occurrences that I found in a computer search where there is no definite article/definite-preposition preceding yom + definite article + adjective: Gen 1:31; 2:3; Ex 12:15, 18; 20:10//Deut 5:14; Lev 19:6; 22:27; Lev 14:57; Josh 24:31; Judg 2:7. In the last three passages the adjective is not a number. For a few other similar cases see BDB 2b: https://archive.org/details/hebrewenglishlex00browuoft/page/208/mode/2up. See p. 209.

(2) It is also rare for there to be a definite-article + yom + definite-article + adjective (i.e., adjectiveהיום ה). I only found four occurrences and they all (coincidentally?) have rishon(im) ("first)" or acharon ("last") for the adjective: Num 6:12; Eccl 7:10; Dan 10:12; Neh 8:18. Instead, the common pattern to express definiteness for yom in the HB is a definite-preposition + yom + definite-article + adjective. I found 117 verses like this. For example, בַּיּוֹם הַשְּׁבִיעִי ("on the seventh day") in Gen 2:2. There is probably nothing too significant about this since it would be more common to speak about a particular day using a preposition than not. Nevertheless, notice that it is even less rare for there to be adjectiveהיום ה than adjectiveיום ה.

Notice that both have the adjectiveיום ה construct phrase but only the first passage has the acrostic and it is broken into two thoughts. If the acrostic were intentional then it seems to me that it would be in a passage like 2:3a since it would be continuous with itself. It would also parallel elohim in the same verse. Because it is not, I'm inclined to think that the acrostic is not intentional.

(1) יוֹם הַשִּׁשִּׁי (yom hash-shish-shi) is a construct phrase where the adjective שִׁשִּׁי acts as a noun. The phrase literally means "the day of the sixth" or "the sixth's day" but can simply be understood as "the sixth day". This kind of construct where the last word is an adjective is not ungrammatical, but it is relatively rare. Here are the only eleven verses that I found in a computer search where there is no definite-article/definite-preposition preceding yom + definite article + adjective (i.e., adjectiveיום ה): Gen 1:31; 2:3; Ex 12:15, 18; 20:10//Deut 5:14; Lev 19:6; 22:27; Lev 14:57; Josh 24:31; Judg 2:7. In the last three passages the adjective is not a number. For a few other similar cases see BDB 2b: https://archive.org/details/hebrewenglishlex00browuoft/page/208/mode/2up. See p. 209.

(2) It is also relatively rare for there to be a definite-article + yom + definite-article + adjective (i.e., adjectiveהיום ה) though this kind of adjectival construction (i.e., adjectiveה nounה) is common overall. I only found four occurrences and they all (coincidentally?) have rishon(im) ("first)" or acharon ("last") for the adjective: Num 6:12; Eccl 7:10; Dan 10:12; Neh 8:18. Instead, the common pattern to express definiteness for yom in the HB is a definite-preposition + yom + definite-article + adjective. I found 117 verses like this. For example, בַּיּוֹם הַשְּׁבִיעִי ("on the seventh day") in Gen 2:2. There is probably nothing too significant about this since it would be more common to speak about a particular day using a preposition than not. Nevertheless, it is interesting to note that adjectiveהיום ה is even more rare than adjectiveיום ה.

Notice that both have the adjectiveיום ה construct phrase but only the first passage has the acrostic and it is broken into two thoughts. If the acrostic were intentional then it seems to me that it would be in a passage like 2:3a since it would be continuous with itself. It would also parallel elohim in the same verse. Because it is not, I'm inclined to think that the acrostic is not intentional.

(4) Perhaps Nahum Sarna is on the right track to suggest that the definiteness of the construct phrase for days six and seven might be due to emphasize these culminating days of creation. I also suggest that perhaps style could explain the choice of the definite construct phrase rather than the definite adjectival phrase. Notice that seven of the eleven adjectiveיום ה verses cited above are considered to be from P: Gen 1:31; 2:3; Ex 12:15, 18; Lev 14:57; 19:6; 22:27. (Ex 20:20 // Deut 5:14 might also be from a priestly editorial hand.) And one of the four adjectiveה nounה verses is considered to be from P: Num 6:12. Thus at least for the word yom there could be a preference in P for the adjectiveיום ה construction where a definite preposition is not used. Just a thought.