Rav Hirsch and the others describe the "Vayehi erev" of each day as a special command to cause each day to stop and the next day to start independently. That is, each day of creation stopped, not at the end of a set period of time, but by an explicit command of Hashem. The next day did not start based on time passing, but as a result of Hashem declaring it a new day.
That is why each day except the sixth is declared as "a second day", "a third day". etc. Only at the end do we have "the sixth day" because this is the start of the continuing natural process. At the end of the sixth day, with the finishing of creation, the next day happened "naturally" and as such did not require a command to cause the day to change.
That is why we have
2 And God completed on the seventh day His work that He did, and He
abstained on the seventh day from all His work that He did.
ב וַיְכַל אֱלֹהִים בַּיּוֹם הַשְּׁבִיעִי מְלַאכְתּוֹ אֲשֶׁר עָשָׂה
וַיִּשְׁבֹּת בַּיּוֹם הַשְּׁבִיעִי מִכָּל מְלַאכְתּוֹ אֲשֶׁר עָשָׂה:
Hashem allowed the seventh day to occur "naturally" and declared that creation had been completed with the beginning of the natural cycle.