"And G-d saw that it was good. And it was evening, and it was morning, one day."
On the first day, Hashem created time. He assigned a twenty-four hour period to day and night. (Rashi) He commanded the light, "Your dominion is by day!" To the darkness, He said, "Your dominion is by night!" (Bereishit Rabbah)
Not, "the first day" because there is no "first" without a "second," which had not yet eventuated. (Ramban)
Earthly and human measurement of time, by a clock of human manufacture, cannot apply to the first three days, as the sun was not then in existence.
"And it was evening, and it was morning, one day" - "One" (and not first) is employed because Yisrael was destined to proclaim G-d's unity with that word, as we read, "Hear O Yisrael, Hashem our G-d, Hashem is One" (Devarim 6:2). For that reason the first day is called one day, because whoever asserts the unity (oneness) of His Name acknowledges that He is the Creator of His world, blessed be He and blessed be His Name.
Source: Midrash Says; Torah Shleimah
