A day is a period of time marked by the earth’s rotation on its axis. This can mean two things:
the full cycle between two sunrises, or
only the part of that cycle when the sun is visible. The other part of the cycle is called “night.”
The word day appears over 2,000 times in the Old Testament and over 350 times in the New Testament. The Hebrew word for “day” is used in many ways, not only in the literal sense. In Hebrew life, a day began in the evening and ended the next evening. This pattern was based on the Torah (compare Genesis 1:14, 19). A literal twenty-four-hour day is often called a civil day.
Other nations in the ancient Near East started their days at different times.
Greek custom was the same as the Hebrews, beginning in the evening.
Babylonians began their day at sunrise.
Egyptians and Romans counted their day from one midnight to the next.
Days and Weeks in the Bible
People commonly recognized parts of the 12-hour day as morning, noon, and evening (Psalms 55:17). They sometimes used terms like:
dawn (Job 3:9),
the heat of the day (1 Samuel 11:11),
noon (Genesis 43:16),
the cool of the day (3:8), and
evening (Ruth 2:17).
The Hebrew phrase "at twilight." (Exodus 12:6) likely meant dusk, the dark part of twilight (Exodus 16:12). People did not divide days into consecutive hours until the time of Christ. The closest Old Testament equivalent was dividing the day into quarters (Nehemiah 9:3). These divisions are similar to the preexilic division of the night into "watches."
The ancient Hebrews did not give names to the days of the week, except for the Sabbath. Instead, they counted them with numbers. This practice continued into New Testament times (Luke 24:1). Because the Sabbath was so important, the Jewish people needed to know the exact time when it began. The Pharisees taught that the Sabbath started when three stars could be seen in the sky after sunset.
The Days of Creation
Many people believe that the days mentioned in the Genesis creation story were twenty-four-hour periods. They use the phrase "there was evening, there was morning" to support this idea. However, this phrase is a Sumerian figure of speech. It pairs opposites—“evening” and “morning”—to show totality (completeness). In this way, “evening-morning” means a full phase of time within God’s work of creation. It stresses the completeness of each stage, not the exact length of time it took.
The Sumerians considered a day to be only the visible 12-hour period. Other nations had a legal day that lasted 24 hours, or a "double day." If the early Genesis writings reflect Sumerian culture, the term "evening-morning" would not match today's idea of a civil day. Instead, it would refer to a phase or general time period.
Hebrew Bible (Old Testament)
In the Old Testament, the word day is often used in a figurative way. For example:
the "day of trouble" (Psalm 20:1), and
the "day of God's wrath" (Job 20:28).
The plural form sometimes describes a king's reign (1 Kings 10:21) or a person's lifespan (Genesis 5:4; 1 Kings 3:14; Psalm 90:12). The book of Daniel calls God the "Ancient of Days" (Daniel 7:9, 13).
In addition to the Sabbath (Genesis 2:3; Exodus 20:8–11), which was set apart for rest and worship, the word day was also used for certain holy celebrations. These included the Passover in the spring (Exodus 12:14; Leviticus 23:5) and the Day of Atonement each autumn (Leviticus 16:29–31). On these special days, people did not work but instead followed the required religious practices.
The New Testament
In the New Testament, the word day was often used in ways similar to Hebrew and other related Semitic languages. At the same time, the division of the night into four military watches came from Greek and Roman custom. The idea of a twelve-hour day in New Testament times came from Babylonian astronomy (compare John 11:9).
Besides using day literally, New Testament authors sometimes used it figuratively. Examples include:
the "day of salvation" (2 Corinthians 6:2) and
the "day of Christ Jesus" (Philippians 1:6).
They also described specific time periods, like the "days of his service" (Luke 1:23). Special feasts mentioned include:
the Days of Unleavened Bread (Acts 12:3), and
the Day of Pentecost (2:1).
In the Old Testament, a human life is described in terms of days (John 9:4). Christians are called "sons of the light and sons of the day" (1 Thessalonians 5:5). Longer periods or eras are also called days (2 Corinthians 6:2; Ephesians 5:16; 6:13; Hebrews 5:7). Hebrew prophets warned about a day of judgment. The New Testament also emphasizes a final divine judgment day when Jesus, the Son of Man, will reveal himself as Lord (Luke 17:30; John 6:39–44; 1 Corinthians 5:5; 1 Thessalonians 5:2; 2 Peter 2:9; 3:7, 12; 1 John 4:17; Revelation 16:14). The "day of eternity" is when time will turn into eternity (2 Peter 3:18). The new Jerusalem, where God's people will live, is described as a place of endless day (Revelation 21:25).