1 Chronicles 24:19
These were the orderings of them in their service to come into the house of the LORD, according to their manner, under Aaron their father, as the LORD God of Israel had commanded him. These were the orderings of them in their service to come into the house of the LORD, according to their manner, under Aaron their father, as the LORD God of Israel had commanded him.
The priests were set in distinct orderings for service, organized by courses rather than acting at random. Each course had its assigned time to come into the house of the Lord and take up its appointed work there. That work was performed according to its established manner under Aaron their father, exactly as the Lord God of Israel had commanded him.