Albert Barnes Commentary Nehemiah 13:19

"And it came to pass that, when the gates of Jerusalem began to be dark before the sabbath, I commanded that the doors should be shut, and commanded that they should not be opened till after the sabbath: and some of my servants set I over the gates, that there should no burden be brought in on the sabbath day." — Nehemiah 13:19 (ASV)

The gates were closed at the sunset of the day before the Sabbath, since the Sabbath was regarded as commencing on the previous evening.