Why does the Torah write "Yom Echad" (one day) and not "Yom Rishon" (the first day), as it does on all the other days?
Rashi: This hints to the fact that it was only on the first day of the Creation that G-d was One - as the angels were not created until the second day.
Ramban (citing the Ibn Ezra): What the Pasuk means is that it was Erev and Boker of one day.
Ramban: The concept of 'first' only exists when there is a second (such as in a sequence of days, objects or events), in which case the first one takes precedence over the second one, either in time or in some other way. Whereas here, the second day did not yet been exist. 1
See R. Chavel's notes on the Ramban, where he queries the author. It is worth noting however, that the Ramban deliberately states 'the second had not yet been made' - and that consequently, there was no sequence on which to base the second.
What is the significance of the statement "And Hashem called the light 'day' ... "?
Ramban (citing the Ibn Ezra): Now that time was created, Hashem arranged the measure of day and the measure of night. And since Adam, who will eventually allot names to the various creations, has not yet been created, Hashem carried out that task here. 1
Ramban: The naming refers to fixing the distinction between the two elements 'Your border (with reference to the sun) extends until here, and your border (with reference to the moon), until here' (Midrash Rabah). 2
Targum Yonasan: (In the same vein as the Ramban) 'Hashem called the light day - for the world's inhabitants to work; and darkness, night - for them to rest.
Rashbam: Just as a king gives a new name to a subject whom he appoints to a high position (as we find by Pharaoh and Yosef), so too, did Hashem give a name to the light and darkness that He just created 3 . Consequently, from now on, whenever we find the term 'night and day' (for example, see Parshas Noach, Bereishis 8:22), it always refers to light and darkness that Hashem created on the first day 4 .
Seforno: Even though the times of light and darkness did not apply in the same way as they do nowadays (seeing as the sun and moon had not yet been placed in the sky). 5
The same will apply to the naming of the sky (in Pasuk 8) and to that of the dry land and the water (in Pasuk 10). Whereas Adam named the living creatures exclusively.
The same will apply to the two other locations that we cited in 1.
What we wrote in the previous note extends to this note as well.
See Seforno who, it appears, disagrees with the Rashbam's explanation.
In the previous Pasuk, commenting on Hashem's division between light and darkness, the Seforno wrote 'During that time, when the initial light served, they were times of light and of darkness, which did not operate by means of the revolving planets, but by the Will of Hashem ... '.
Why did Hashem call the evening "Erev" and the morning "Boker"?
Ramban: Hashem call the evening "Erev", from the word 'Le'arev' (to mix) - because in the evening the shapes of all things mix together; and He called the morning "Boker", from the word 'Levaker' (to distinguish) - because that is when one can distinguish between them.
רש"י: למה כתב אחד וכו': אולי קודם שנבראו שאר הימים אי אפשר לומר "יום ראשון" אלא יום אחד?
גור אריה: בזמן שכתב משה את התורה כבר נבראו שאר הימים ואפשר היה לכתוב "יום ראשון".
גור אריה: אמנם לא הושלמו שאר הימים [ולא יצאו אל הפועל] אבל הם כבר נבראו [והם קיימים בכח] שהרי ששת ימי בראשית ענין אחד הם, ולכן אפשר לכתוב "יום ראשון".
רש"י: שהיה...יחיד בעולמו: כיצד יתפרש הפסוק לפי זה?
גור אריה: אין הכוונה לפרש את הפסוק 'יום שהיה הקב"ה אחד' אלא יום אחד כמשמעו, ונקרא אחד ולא ראשון ע"ש שבאותו יום היה הקב"ה אחד בעולמו, עיי"ש.