Our convention of starting a new day at midnight is an arbitrary, humanly devised practice. God, who created the heavenly bodies and set them in motion to mark the passage of time (Genesis 1:14 Genesis 1:14And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
American King James Version×), counts time differently—from evening to evening.

We see this indicated in the creation account in Genesis 1. After dividing day from night, God tells us that "the evening and the morning were the first day" (Genesis 1:5 Genesis 1:5And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
American King James Version×). "Evening" is mentioned first, followed by "morning." God describes each day's creation in similar terms (Genesis 1:8 Genesis 1:8And God called the firmament Heaven. And the evening and the morning were the second day.
American King James Version×, Genesis 1:13 Genesis 1:13And the evening and the morning were the third day.
American King James Version×, Genesis 1:19 Genesis 1:19And the evening and the morning were the fourth day.
American King James Version×, Genesis 1:23 Genesis 1:23And the evening and the morning were the fifth day.
American King James Version×, Genesis 1:31 Genesis 1:31And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day.
American King James Version×).

In the Bible, evening began when the sun went down (Joshua 8:29 Joshua 8:29And the king of Ai he hanged on a tree until eventide: and as soon as the sun was down, Joshua commanded that they should take his carcass down from the tree, and cast it at the entering of the gate of the city, and raise thereon a great heap of stones, that remains to this day.
American King James Version×; 2 Chronicles 18:34 2 Chronicles 18:34And the battle increased that day: however, the king of Israel stayed himself up in his chariot against the Syrians until the even: and about the time of the sun going down he died.
American King James Version×; Nehemiah 13:19 Nehemiah 13:19And it came to pass, that when the gates of Jerusalem began to be dark before the sabbath, I commanded that the gates should be shut, and charged that they should not be opened till after the sabbath: and some of my servants set I at the gates, that there should no burden be brought in on the sabbath day.
American King James Version×; Mark 1:32 Mark 1:32And at even, when the sun did set, they brought to him all that were diseased, and them that were possessed with devils.
American King James Version×), and at that time a new day began. Regarding His Sabbaths, God commands that they be observed "from evening to evening" (Leviticus 23:32 Leviticus 23:32It shall be to you a sabbath of rest, and you shall afflict your souls: in the ninth day of the month at even, from even to even, shall you celebrate your sabbath.
American King James Version×). This was the usual way at that time of calculating the beginning and ending of days (Exodus 12:18 Exodus 12:18In the first month, on the fourteenth day of the month at even, you shall eat unleavened bread, until the one and twentieth day of the month at even.
American King James Version×).

In New Testament times, days were calculated the same way. Mark 1:32 Mark 1:32And at even, when the sun did set, they brought to him all that were diseased, and them that were possessed with devils.
American King James Version×records that, after the sun had set, marking the end of one Sabbath, crowds brought many ailing people to Jesus to be healed, having waited until after the Sabbath to come to Him. The Gospel accounts also record that Joseph of Arimathea entombed Jesus' body before evening to keep from working on an approaching annual high-day Sabbath (Matthew 27:57-60 Matthew 27:57-60 [57] When the even was come, there came a rich man of Arimathaea, named Joseph, who also himself was Jesus' disciple: [58] He went to Pilate, and begged the body of Jesus. Then Pilate commanded the body to be delivered. [59] And when Joseph had taken the body, he wrapped it in a clean linen cloth, [60] And laid it in his own new tomb, which he had hewn out in the rock: and he rolled a great stone to the door of the sepulcher, and departed.
American King James Version×; Mark 15:42-46 Mark 15:42-46 [42] And now when the even was come, because it was the preparation, that is, the day before the sabbath, [43] Joseph of Arimathaea, an honorable counselor, which also waited for the kingdom of God, came, and went in boldly to Pilate, and craved the body of Jesus. [44] And Pilate marveled if he were already dead: and calling to him the centurion, he asked him whether he had been any while dead. [45] And when he knew it of the centurion, he gave the body to Joseph. [46] And he bought fine linen, and took him down, and wrapped him in the linen, and laid him in a sepulcher which was hewn out of a rock, and rolled a stone to the door of the sepulcher.
American King James Version×; Luke 23:50-54 Luke 23:50-54 [50] And, behold, there was a man named Joseph, a counselor; and he was a good man, and a just: [51] (The same had not consented to the counsel and deed of them;) he was of Arimathaea, a city of the Jews: who also himself waited for the kingdom of God. [52] This man went to Pilate, and begged the body of Jesus. [53] And he took it down, and wrapped it in linen, and laid it in a sepulcher that was hewn in stone, wherein never man before was laid. [54] And that day was the preparation, and the sabbath drew on.
American King James Version×; compare John 19:31 John 19:31The Jews therefore, because it was the preparation, that the bodies should not remain on the cross on the sabbath day, (for that sabbath day was an high day,) sought Pilate that their legs might be broken, and that they might be taken away.
American King James Version×).

God, Creator of the Sabbath, determines when the day begins and ends, and it was observed from sunset to sunset throughout the Bible. His Sabbath begins Friday evening at sunset and ends Saturday evening at sunset.