“Its gates shall not be shut at all by day (there shall be no night there).”
Another continual comparison to ancient Jerusalem (the type/shadow). The gates of the old city were closed on sabbath days (Neh.13:19) and the temple gates were always closed at night, (Horn, Siegriud H., Seventh-Day Adventist Bible Dictionary, (Review and Herald, 1960), “Council,” page 228. ) but the gates of the new Jerusalem are never closed. It is always day for, “there shall be no night there.” The new city may be entered at any time by those written in the Lamb’s Book of Life.
[For a more in-depth study see eschatology “Study Series 16 Lesson 9 Rev. Chapter 21”]