The Jews reckoned the day from sunset to sunset (Leviticus 23:32). It was originally divided into three parts (Psalm 55:17). "The heat of the day" (1 Samuel 11:11; Nehemiah 7:3) was at our nine o'clock, and "the cool of the day" just before sunset (Genesis 3:8). Before the Captivity the Jews divided the night into three watches, (1) from sunset to midnight (Lamentations 2:19); (2) from midnight till the cock-crowing (Judges 7:19); and (3) from the cock-crowing till sunrise (Exodus 14:24). In the New Testament the division of the Greeks and Romans into four watches was adopted (Mark 13:35). (see WATCHES.)

The division of the day by hours is first mentioned in Dan. 3:6, 15; 4:19; 5:5. This mode of reckoning was borrowed from the Chaldeans. The reckoning of twelve hours was from sunrise to sunset, and accordingly the hours were of variable length (John 11:9).

The word "day" sometimes signifies an indefinite time (Genesis 2:4; Isaiah 22:5; Hebrews 3:8, etc.). In Job 3:1 it denotes a birthday, and in Isaiah 2:12, Acts 17:31, and 2 Timothy 1:18, the great day of final judgment.

Noah Webster's New International Dictionary of the English Language

1. (n.) The time of light, or interval between one night and the next; the time between sunrise and sunset, or from dawn to darkness; hence, the light; sunshine.

2. (n.) The period of the earth's revolution on its axis. -- ordinarily divided into twenty-four hours. It is measured by the interval between two successive transits of a celestial body over the same meridian, and takes a specific name from that of the body. Thus, if this is the sun, the day (the interval between two successive transits of the sun's center over the same meridian) is called a solar day; if it is a star, a sidereal day; if it is the moon, a lunar day. See Civil day, Sidereal day, below.

3. (n.) Those hours, or the daily recurring period, allotted by usage or law for work.

4. (n.) A specified time or period; time, considered with reference to the existence or prominence of a person or thing; age; time.

5. (n.) (Preceded by the) Some day in particular, as some day of contest, some anniversary, etc.