4 Now this is what you are to do: A third of you priests and Levites who are going on duty on the Sabbath are to keep watch at the doors,

Read 2 Chronicles 23:4 Using Other Translations

This is the thing that ye shall do; A third part of you entering on the sabbath, of the priests and of the Levites, shall be porters of the doors;
This is the thing that you shall do: of you priests and Levites who come off duty on the Sabbath, one third shall be gatekeepers
This is what you must do. When you priests and Levites come on duty on the Sabbath, a third of you will serve as gatekeepers.