Jamieson, Fausset and Brown on 2 Chronicles 29:12-19

12-19. Then the Levites arose--Fourteen chiefs undertook the duty of collecting and preparing their brethren for the important work of cleansing the Lord's house. Beginning with the outer courts--that of the priests and that of the people--the cleansing of these occupied eight days, after which they set themselves to purify the interior; but as the Levites were not allowed to enter within the walls of the temple, the priest brought all the sweepings out to the porch, where they were received by the Levites and thrown into the brook Kedron. This took eight days more. At the end of this period they repaired to the palace and announced that not only had the whole of the sacred edifice, within and without, undergone a thorough purification, but all the vessels which the late king had taken away and applied to a common use in his palace, had been restored, "and sanctified."