parshanut torah comment - "And it was evening, and it was morning, *one* day"?

Gen. 1:5:

וַיְהִי עֶרֶב וַיְהִי בֹקֶר יוֹם אֶחָד

And it was evening, and it was morning, one day

Gen. 1:8:

וַיְהִי עֶרֶב וַיְהִי בֹקֶר יוֹם שֵׁנִי

and it was evening, and it was morning, a second day.

1. Why would the Torah write "יום אחד" (a cardinal number, instead of an ordinal number, h/t Danno) for the first day?


2. Why shouldn't that reason apply to the second (and subsequent) days?

Answer

Rashi says it is "the day of the One". Because the angels weren't created until day two, God was the only sentient being on this day. Obviously this reason doesn't apply from day two onwards, per Rashi's words.

Kli Yakar prefers to say that the verse is asserting that one God created both light/day, and dark/evening, as the Sages would take care to mention one alongside the other. This was so that heretics would not conclude that some other entity created darkness and night when confronted with two opposite states stemming from one creative act.

Ramban, (in addition to what he says in user6591's answer), says that this day was actually one day, as opposed to the fourth day, when the existence of the sun caused there to be 'many' days and nights, in a 24-hour period, depending on location on the earth, and the earth's rotation.