C. F. Keil and Franz Delitzsch on Nehemiah 12:30

Neh 12:30

The dedication began with the purification of the people, the gates, and the wall, by the priests and Levites, after they had purified themselves. This was probably done, judging from the analogy of 2Ch 29:20, by the offering of sin-offerings and burnt-offerings, according to some special ritual unknown to us, as sacrifices of purification and dedication. This was followed by the central-point of the solemnity, a procession of two bands of singers upon the wall (Neh 12:31-42).