Verse 19. These, &c. - In this order and method they were to come to perform the offices of the temple. To come - To come into the temple every sabbath-day, and to continue there 'till the next sabbath, when they were relieved by others. Aaron - Under the direction of the high-priests, whom he calls Aaron, because he represented his person and executed his office, and their father, because of the authority which by God's appointment he had over them.
Verse 31. Over against - Answerable for number and order to those of the priests, so that there should be a course of the Levites for each course of the priests.