Shabbat can, in a certain sense, be viewed as one unified whole.
The Midrash says that the all-encompassing light that burst forth on the first day of Creation became increasingly bright in the course of the First Shabbat.
Thus, Shabbat can be described as a day in which there is no darkness, only light.
Therefore, on Shabbat people do not greet each other with "Good evening" or "Good morning," as ordinarily, but rather with "Shabbat Shalom" ("A good Shabbat," or "A Shabbat of peace").
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer, p. 105, by Rabbi Adin Steinsaltz