Ramban

AND G-D CALLED THE LIGHT DAY. The verse states that time was created, and G-d made the length of the day and the length of the night. The purport of the word vayikra (And He called) is [to indicate that] since Adam later gave names [to all the beasts, the fowl, etc.], it states that those things which were made before his existence were given names by G-d. This is the opinion of Rabbi Abraham ibn Ezra. The correct interpretation is that the matter of calling a name here indicates the division which bounded them when they assumed their form. Thus did the Rabbis say: “[G-d said to the light,] ‘The day shall be your boundary,’ [and to darkness He said,] ‘The night shall be your boundary.’” AND THERE WAS EVENING AND THERE WAS MORNING. There was evening and there was morning of one day. The beginning of the night is called erev [which also means “mingling”] because shapes of things appear confused in it, and the beginning of the day is called boker [which also means “examining”] because then a man can distinguish between various forms. This coincides with the explanation of Rabbi Abraham ibn Ezra. By way of the simple explanation of Scripture, it could not have said, [And there was evening and there was morning] “the first day,”. See Rashi who says that according to the regular mode of expression it should have really said “the first day.” He explains, however, the expression “one day” on the basis of the Midrash: It is “because the Holy One, blessed be He, was then the Only One (the Sole Being) in His universe, since the angels were not created until the second day.” One day thus means “the day of the One Being.” It is this interpretation of Rashi that Ramban alludes to when he comments that according to the simple meaning of Scripture it could not have said, “the first day.” because the second had not yet been made; “the first” precedes a “second” in number or degree but both exist, whereas “one” does not connote the existence of a second. Some scholars explain that one day is a reference to the rotation of the sphere upon the face of the whole earth in twenty-four hours, as every moment thereof is morning in some different place and night in the opposite place. thus means that the entire day consists of evening and morning occurring simultaneously in different places. If so, the verse alludes to that which will take place in the firmament after the luminaries will be placed in the firmament of the heavens.