These are the sons of Levi twenty years and older, divided up according to families and heads of families and listed in the work groups that took care of the worship in the sanctuary of God.
David said, "Now that the God of Israel has given rest to his people and made Jerusalem his permanent home,
the Levites no longer have to carry the Tabernacle and all the furniture required for the work of worship."
These last words of David referred only to Levites twenty years old and above.
From now on the assigned work of the Levites was to assist Aaron's sons in the work of worship in God's house: maintain courtyards and closets, keep the furniture and utensils of worship clean, take care of any extra work needed in the work of worship,
and provide bread for the table and flour for the Meal Offerings and the unraised wafers - all baking and mixing, all measuring and weighing.
Also they were to be present for morning prayers, thanking and praising God, for evening prayers,
and at the service of Whole-Burnt-Offerings to God on Sabbath, at New Moons, and at all festivals. They were on regular duty to serve God according to their assignment and the required number.
In short, the Levites, with the sons of Aaron as their companions in the ministry of holy worship, were responsible for everything that had to do with worship: the place and times and ordering of worship.