The most natural explanation for a day being measured from evening to morning in Genesis 1 is that the beginning of time was marked by darkness. Genesis 1:2 notes, “The earth was without form and void, and darkness was over the face of the deep.”
Then, in Genesis 1:3-5, “God said, ‘Let there be light,’ and there was light. And God saw that the light was good. And God separated the light from the darkness. God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.” Light came after darkness, indicating the markers of a full day. In the Jewish tradition, days follow this pattern of night first, then day. Still today, practicing Jews observe the Sabbath beginning on sunset of Friday until sunset on Saturday.
Eventually, there was a change to today’s practice where a day starts at midnight. The modern calendar is based on the Gregorian calendar, a revision of the Julian calendar implemented in 45 B.C. by Julius Caesar. As Christianity spread throughout Europe, the Julian calendar became the standard adopted by other nations. The revision into the Gregorian calendar came in 1582.
Genesis 1 reveals that the days of the week were originally measured “evening” then “morning.” Each description of the seven days of creation uses this format, clearly indicating a day that began at sunset.
The Jews in the time of Jesus continued to recognize this pattern. One example in the Gospels involves the burial of Jesus. Joseph of Arimathea placed Jesus in a nearby tomb just before sunset (John 19:42). Luke 23:54 reads, “It was the preparation day, and the Sabbath was about to begin” (NASB). The fact that the Sabbath began at sundown also explains why the two thieves’ deaths were hastened (John 19:31).
Genesis 1 marks days from evening to morning as a natural extension of God turning darkness into light. This tradition continued through the New Testament period and is practiced today by many observant Jews. Christians are not bound to divide days the same way, but an understanding of the Jewish way of reckoning time is basic to understanding some of the cultural practices in the Bible.