The Sabbath begins Friday evening at sunset and ends Saturday evening at sunset.
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), 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" (verse 5). "Evening" is mentioned first, followed by "morning." God describes each day's creation in similar terms (verses 8, 13, 19, 23, 31).
In the Bible, evening began when the sun went down (Joshua:8:29; 2 Chronicles:18:34; Nehemiah:13:19; Mark:1:32), and at that time a new day began. Regarding His Sabbaths, God commands that they be observed "from evening to evening" (Leviticus:23:32). This was the usual way at that time of calculating the beginning and ending of days (Exodus:12:18).
In New Testament times, days were calculated the same way. Mark:1:32 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; Mark:15:42-46; Luke:23:50-54; compare John:19:31).
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.