Charles Ellicott Commentary 1 Chronicles 24:19

"This was the ordering of them in their service, to come into the house of Jehovah according to the ordinance [given] unto them by Aaron their father, as Jehovah, the God of Israel, had commanded him." — 1 Chronicles 24:19 (ASV)

These were the orderings of them in their service. —Better, These were their classes for their service.

According to their manner, under Aaron. —Better, according to their rule (or order; Vulgate, ritum), ordained through Aaron, and so forth. (Numbers 4:1; Numbers 4:17.) All the sacerdotal functions were established, and each of the twenty-four classes undertook their weekly discharge in rotation with the others, beginning on the Sabbath (2 Kings 11:9; 2 Chronicles 23:8). Josephus (Ant. vii. 14, 7) declares that David’s arrangements continued until his own day.

1. Recapitulation of the Levitical classes (1 Chronicles 24:20–31). (Compare 1 Chronicles 23:12–23).