Genese 1:5 Hieronymus traduxit:
Appellavitque lucem Diem, et tenebras Noctem: factumque est vespere et mane, dies unus.
Cur "unus", non "primus"? Nonne numerum ordinalem significat? Nonne "unus" est cardinalis?
This is a literal translation of the Hebrew original, which has yom ehad "one day", not yom rishon "first day".
It's a cardinal number, not an ordinal number, in the original Hebrew. Look at the other uses of the same Hebrew phrase:
And God said, “Let the water under the sky be gathered to one place, and let dry ground appear.” And it was so.
That is why a man leaves his father and mother and is united to his wife, and they become one flesh.
And so on. The vast majority are clearly 'one'.
The NAS translates Gen 1:5 literally as "one day":
God called the light day, and the darkness He called night. And there was evening and there was morning, one day.