Jamieson, Fausset and Brown on Genesis 1:3

Ge 1:3-5. The First Day.

3. God said--This phrase, which occurs so repeatedly in the account means: willed, decreed, appointed; and the determining will of God was followed in every instance by an immediate result. Whether the sun was created at the same time with, or long before, the earth, the dense accumulation of fogs and vapors which enveloped the chaos had covered the globe with a settled gloom. But by the command of God, light was rendered visible; the thick murky clouds were dispersed, broken, or rarefied, and light diffused over the expanse of waters. The effect is described in the name "day," which in Hebrew signifies "warmth," "heat"; while the name "night" signifies a "rolling up," as night wraps all things in a shady mantle.