and gave them the following orders: "When you come on duty on the Sabbath, one third of you are to guard the palace;

The two groups that go off duty on the Sabbath are to stand guard at the Temple to protect the king.

The Levites and the people of Judah carried out Jehoiada's instructions. The men were not dismissed when they went off duty on the Sabbath, so the commanders had available both those coming on duty and those going off.