And God named...
The verb קָרָא (qara) means to address by naming. When God gives a name to something he defines its primary purpose or function. It is away to show sovereignty over that which is named.
Light is named Day
By declaration, Light was called upon to be a unit of time. In reference to the earth, the daylight would be that part in which the lightbearer would light the inhabitants. The word יֹ֔ום (yom) does not denote an exact period of time, but rather the nature of that time.
וְלַחֹ֖שֶׁךְ קָ֣רָא לָ֑יְלָה
And the darkness He called Night
The naming of the darkness transformed chaos into a time for rest. The word לַיִל (layil) is a primitive one, but seems to have been derived from לוּל (lul), which means to fold back, that is, to turn around.
In the naming of the periodic darkness upon the earth, God leaves a clue to how He "separated" the darkness. It was by turning the surface away from the light, blocking it for a time.