Genesis 1:5 says, “The evening and the morning were the first day.” A day began when the preceding day closed, at sunset. The dark part of the day came first, then the light part.
“Evening” is when the sun becomes even with the horizon, what we call sunset. “In the evening, at the going down of the sun …” (Deuteronomy 16:6). “That evening, after sunset …” (Mark 1:32).
Also, we read that the prophet Nehemiah after the Babylonian captivity taught the Israelites how to keep the Sabbath by ordering the gates of Jerusalem to be shut to keep the Sabbath holy “when the evening shadows fell on the gates of Jerusalem before the Sabbath” (Nehemiah 13:15-19).