John Gill 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: Their classes or courses into which they were divided, in their service to come into the house of the Lord; that is, to perform their service in the temple, according to their turns, and in course as fixed by lot. One class or course served one week, and then went out, and another came in, each consisting of 1000 men (see 1 Chronicles 23:4).

This was according to their manner, meaning the law, right, or custom here prescribed for them. This service was performed under Aaron their father; that is, under him who was high priest for the time being. He bore this name because he was Aaron's successor and representative, under whose inspection, direction, and authority, all the rest of the priests were, and whom they were to obey in all things.

This was done as the Lord God of Israel had commanded him; that is, commanded him to take care they should be done.