1 Chronicles 24:1

Now these are the divisions of the sons of Aaron. The sons of Aaron; Nadab, and Abihu, Eleazar, and Ithamar.

This verse opens the listing of the divisions of the sons of Aaron for priestly service. It marks the formal organization of Aaron's descendants into set orders for carrying on the work of the priesthood. The priests were arranged into twenty-four courses. Each course was placed under its own chief. A full and accurate record of these divisions was kept.

Each course served in turn at the sanctuary in Jerusalem. The term of service for each course was one week at a time. Each course thus ministered twice in the year.