See the marginal references and notes. 1-Chronicles 23:28-32 give the most complete account in Scripture of the nature of the Levitical office.

David was old and full of days - On the phrase full of days, see the note on Genesis 25:8.

So when David was old and full of days,.... Perhaps was now in the last year of his age, about seventy years old, though before he was bedridden; see 1-Chronicles 28:2,
he made Solomon his son king over Israel; declared him to be his successor; this was before the affair of Adonijah, for then he ordered him to be anointed king, and placed on the throne; and this aggravated the rebellion of Adonijah, that it was against the declared and known will of his father.

David, having given charge concerning the building of the temple, settles the method of the temple service, and orders the officers of it. When those of the same family were employed together, it would engage them to love and assist one another.

DAVID MAKES SOLOMON KING. (1-Chronicles 23:1)
when David was old . . . he made Solomon . . . king--This brief statement, which comprises the substance of 1Ki. 1:32-48, is made here solely to introduce an account of the preparations carried on by David during the latter years of his life for providing a national place of worship.

Number, duties, and fathers'-houses of the Levites. - This clear account of the state and the order of service of the tribe of Levi is introduced by the words, 1-Chronicles 23:1, "David was old, and life weary; then he made his son Solomon king over Israel." זקן, generally an adjective, is here third pers. perf. of the verb, as in Genesis 18:12, as שׂבע also is, to which ימים is subordinated in the accusative. Generally elsewhere ימים שׂבע is used, cf. Genesis 35:29; Job 42:17, and also שׂבע alone, with the same signification, Genesis 25:8. These words are indeed, as Berth. correctly remarks, not a mere passing remark which is taken up again at a later stage, say 1-Chronicles 29:28, but an independent statement complete in itself, with which here the enumeration of the arrangements which David made in the last period of his life begins. But notwithstanding that, it serves here only as an introduction to the arrangements which follow, and is not to be taken to mean that David undertook the numbering of the Levites and the arrangement of their service only after he had given over the government to his son Solomon, but signified that the arrangement of this matter immediately preceded Solomon's elevation to the throne, or was contemporaneous with it. Our verse therefore does not contain, in its few words, a "summary of the contents of the narrative 1 Kings 1," as Berth. thinks, for in 1 Kings 1 we have an account of the actual anointing of Solomon and his accession to the throne in consequence of Adonijah's attempt to usurp it. By that indeed Solomon certainly was made king; but the chronicler, in accordance with the plan of his book, has withdrawn his attention from this event, connected as it was with David's domestic relations, and has used המליך in its more general signification, to denote not merely the actual elevation to the throne, but also his nomination as king. Here the nomination of Solomon to be king, which preceded the anointing narrated in 1 Kings 1, that taking place at a time when David had already become bed-rid through old age, is spoken of. This was the first step towards the transfer of the kingdom to Solomon; and David's ordering of the Levitical service, and of the other branches of public administration, so as to give over a well-ordered kingdom to his successor, were also steps in the same process. Of the various branches of the public administration, our historian notices in detail on the Levites and their service, compressing everything else into the account of the army arrangements and the chief public officials, 1 Chron 27.