Adam Clarke on 2 Chronicles 29:16

And the priests went - The priests and Levites cleansed first the courts both of the priests and of the people. On this labor they spent eight days. Then they cleansed the interior of the temple; but as the Levites had no right to enter the temple, the priests carried all the dirt and rubbish to the porch, whence they were collected by the Levites, carried away, and cast into the brook Kidron; in this work eight days more were occupied, and thus the temple was purified in sixteen days.