1 Chronicles 23:27-31

For by the last words of David, the sons of Levi were counted from twenty years old and upward.

For their office is to assist the sons of Aaron with the service of the house of the Lord, in the courtyards and in the chambers, and in the purification of all holy things, and the work of the service of the house of God,

and with the showbread, and the fine flour for a grain offering, and unleavened wafers, or what is baked in the pan or what is well-mixed, and all measures of volume and size.

They are to stand every morning to thank and to praise the Lord, and likewise at evening,

and to offer all burnt offerings to the Lord, on the Sabbaths, the new moons and the appointed festivals, in the number determined by the ordinance concerning them, continually before the Lord.