Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
See the marginal references and notes. Ch1 23:28-32 give the most complete account in Scripture of the nature of the Levitical office.
From the age of twenty years - The Levites had hitherto not entered upon their regular functions until the age of thirty Ch1 23:3. Certain lighter duties were by the Law imposed on them at the age of twenty-five Num 8:24; but it was not until they were five years older that they became liable to the full service of the sanctuary. David appears now to have made a change. By his "last words" Ch1 23:27 the time for the Levites to enter on the full duties of their office was advanced from thirty to twenty years. This change was based upon the lighter character of the labors imposed on them now that the ark had ceased to be carried from place to place and obtained a permanent habitation Ch1 23:26. The limit of age continued in after times where David had fixed it (see Ezr 3:8).
By the "last words of David" some understand an historical work on the latter part of his reign, drawn up probably by Gad or Nathan (compare Ch1 27:24; Ch1 29:29). Others suppose that he left behind him a work containing directions for the service of the sanctuary.
Though the Levites were not allowed by themselves to offer sacrifice, yet there were many respects in which they assisted the priests when sacrifice was offered. See Ch2 29:34; Ch2 35:11-12.
The set feasts - The Passover, Feast of Pentecost, and Feast of Tabernacles (marginal reference).