The order in which the priests were on duty at the temple. See ABIA.
When David was not permitted to build the temple, he proceeded, among the last acts of his life, with the assistance of Zadok and Ahimelech, to organize the priestly and musical services to be conducted in the house of God. (1.) He divided the priests into twenty-four courses (1Ch 24:1-19), sixteen being of the house of Eleazar and eight of that of Ithamar. Each course was under a head or chief, and ministered for a week, the order being determined by lot. (2.) The rest of the 38,000 Levites (1Ch 23:4) were divided also into twenty-four courses, each to render some allotted service in public worship: 4,000 in twenty-four courses were set apart as singers and musicians under separate leaders (25); 4,000 as porters or keepers of the doors and gates of the sanctuary (1Ch 26:1-19); and 6,000 as officers and judges to see to the administration of the law in all civil and ecclesiastical matters (1Ch 26:20-32).
This arrangement was re-established by Hezekiah (2Ch 31:2); and afterwards the four sacerdotal courses which are said to have returned from the Captivity were re-divided into the original number of twenty-four by Ezra (Ezr 6:18).
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 1Ch 24:6-19. The Levites were divided in a similar manner. 1 Chr. 23. David also instituted in the army a kind of militia, each course to serve a month. 1 Chr. 27.
The courses of the priests and Levites were restored by Ezra on the return from captivity, Ezr 6:18, and we find them still in operation in the N.T. Zacharias the father of John the Baptist was of the course of Abia, which doubtless refers to Abijah, the eighth name mentioned in 1Ch 24:10. At the end of his service he returned to his house. Lu 1:5,23. The length of service was a week, commencing from the Sabbath. 2Ch 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 Re 4:4, etc. representing the complete heavenly priesthood.