Purified themselves ceremonially, then purify the people and the wall

Nehemiah 12:1-47
At the dedication of the wall of Jerusalem, the Levites were sought out from where they lived and were bought to Jerusalem to celebrate joyfully the dedication with songs of thanksgiving and with the music of cymbals harps and lyres.
When the priests and Levites had purified themselves ceremonially, they purified the people the gates and the wall.
Nehemiah assigned two large choirs to give thanks.
Two processions, each led by a choir, make their way in opposite directions along the broad tip of the wall,meeting in the Temple area for the concluding thanksgiving and sacrifices. It is an occasion of tumultuous joy and noisy celebration.
At that time men were appointed to be in charge of the storerooms for the contributions, first fruits and tithes. All Israel contributed the daily portions for the singers and gatekeepers.
Why?
It is not bureaucratic pedantry that has preserved these names The point is that these people and their chronicler are conscious of their roots and of their structure as God's company. This is no rabble烏合之眾 of refugees, settling down anywhere.
All of people of Levites and priests followed the commands of David and Solomon to serve in the temple. Back to worship the Lord with joy and their duty.
How?
Choir is the leader of thanksgiving, and the flight for the Lord. Member of choir and priests need to purify themselves first, them purify the people and other matters. Each Sunday is the Holy day of God.