Verse 1 – The temple service was highly structured. Among all the divisions established at this time. The most important one was for priests, who were divided into 24 segments. These 24 groupings were based on Aaron’s two surviving sons, Eleazar and Ithamar.
Verse 2 – The scripture only mentions that Nadad and Abihu were dead and had no descendents, but avoided sharing the details of them dieing prematurely because they defiled the temple.
Verse 3 – Zadok served the tabernacle at Gibeon and Abiathar was in charge of the ark in Jerusalem.
Verse 4 – Since Eleazar’s descendants outnumbered those of Ithamar by a ratio of two to one, it made sense that of the 24 divisions, Eleazar should get 16 shifts, and Ithamar only eight.
Verses 7-19 – Each of these 24 groups of priests served two-week shifts each year at the temple. The rest of the time they served in their hometowns. This system was still in place in Jesus day (Luke 1:5-9). Zachariah was a member of the Abijah division. During this shift at the temple an angel appeared to him and predicted that he would have a son, John.