Gaebelein's Annotated Bible
Now these are the divisions of the sons of Aaron. The sons of Aaron; Nadab, and Abihu, Eleazar, and Ithamar.3. The Twenty-four Courses of the Priests
1. The twenty-four courses (1Chronicles 24:1-19)
2. The organization of other Levites (1Chronicles 24:20-31)
In the previous chapter we read of 24,000 Levites set apart for the service. In the next chapter we find twenty-four leaders of song and music appointed, and here David instituted twenty-four courses of priests. Each of these ministered a full week, from one sabbath to the next. These courses were not only continued by Solomon, but also by Hezekiah and Josiah. From Luke 1:5, we learn the same order was still followed in the days our Lord was born. Zecharias belonged to the eighth course, that of Abijah.
In the book of Revelation (chapter 4, etc.) we read of twenty-four elders clothed in white raiment, crowned and seated upon twenty-four thrones. They represent symbolically all the redeemed brought into glory. This number is obviously an allusion to the arrangement of the priesthood made by David for the service of the temple under the glorious reign of Solomon, the blessed type of the reign of Christ in glory. As these twenty-four courses of Priests were to minister during Solomon’s glorious reign, they are typical of the redeemed, the holy and royal priesthood, associated with the Lord Jesus Christ when He occupies His throne of glory.