Rashi on the Prophets and Writings

the sons of the Levites were numbered from twenty years old and upwards Then David established these watches, so that when Solomon his son would come, they would be prepared for him.

For their station was alongside the sons of Aaron, etc. This resembles (Num. 18:2): “... and they shall join with you and minister to you.”
and over the purity of all sacred things so that if the House and its vessels become unclean, they will be appointed to purify it, for so we find with Hezekiah, as it is written (below II Chron. 29:16): “And the Levites accepted to take out to the Kidron Valley outside,” and it is written (ibid. verse 17): “And they commenced on the first of the first month to sanctify, etc.”

And for every offering up of burnt offerings to the Lord on Sabbaths, on New Moons The meaning is: They will be appointed over the chambers and also over the chamber of the house of the lambs, where the sacrificial animals are kept, and they will bring [them] to the priests constantly every day, according to the number that is required for each day, whether on weekdays, whether on Sabbaths, and on New Moons and festivals, when additional sacrifices are offered up with the daily offerings - all these they will bring in number according to the ordinance, and they will also assist the priests in flaying the sacrifice. And so we find with Hezekiah (II Chron. 29:34): “Only the priests were few, and they could not flay all the burnt offerings; so their brethren the Levites strengthened them, etc.”