AFTERNOON. The Hebrews reckoned the day from evening to evening and divided it into the following six unequal parts: the break of day; the morning, or sunrise; the heat of the day (beginning about nine o’clock); midday; the cool of the day; and the evening. The cool of the day corresponded to our late afternoon, and was so called because in Eastern countries a wind begins to blow a few hours before sundown and continues till evening. At that time much of the day’s business was transacted. See Genesis 3:8 and Judges 19:9.

International Standard Bible Encyclopedia (1915)

The expression kechom ha-yom, "in the heat of the day" (Ge 18:1) refers to the early afternoon when the sun is a little past its zenith, its rays still being very strong. The phrase le-ruach ha-yom, "in the cool of the day" (Ge 3:8) is in contrast to the last phrase and points to the late afternoon; in the Orient a cooling breeze arises at this period of the day, and it is then that much of the day’s business is transacted.

See Day.