1 Chronicles 24:2
But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest’s office. But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest’s office.
The verse stands within David's organization of the priests into twenty-four courses. Those priestly divisions were carefully recorded and placed under chiefs for orderly service. Each course came up to Jerusalem twice a year to minister at the sanctuary for one week at a time.