Cardinal number

At the end of day one, God says it was day one, using a cardinal number. On the rest of the days, he used ordinal numbers, as second, third, etc.

It is suggestive, as is Ge 2:5, that all of creation happened on one day.

The word for 'one' אחד also means 'go thee one way or the other.

Eze 21:16 Go thee one way or other (אחד), [either] on the right hand, [or] on the left, whithersoever thy face [is] set.

When he called it 'Day one', he said, this is the day to choose whom you will serve.

De 30:19 I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing: therefore choose life, that both thou and thy seed may live:

Jos 24:15 And if it seem evil unto you to serve the LORD, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the LORD.

Heb 4:7 Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.