Boyd’s Bible Dictionary:
(running). Priests divided into twenty-four classes, courses, or orders (1 Chron. 24). [ASIA.]
Concise Bible Dictionary:
David divided the priests into 24 courses: 16 of them were of the house of Eleazar, and 8 of Ithamar. A list of them, under the name of each head, is given in 1 Chronicles 24:6-19. The Levites were divided in a similar manner (1 Chron. 23). David also instituted in the army a kind of militia, each course to serve a month (1 Chron. 27).
The courses of the priests and Levites were restored by Ezra on the return from captivity (Ezra 6:18), and we find them still in operation in the New Testament Zacharias the father of John the Baptist was of the course of Abia, which doubtless refers to Abijah, the eighth name mentioned in 1 Chronicles 24:10. At the end of his service he returned to his house (Luke 1:5, 23). The length of service was a week, commencing from the Sabbath (2 Chron. 23:8).
Twenty-four is a number seldom found in the scripture: there may therefore, as to number, be an allusion to the 24 courses of priests in the 24 elders seated on thrones in Revelation 4:4, representing the complete heavenly priesthood.