And Zadok the priest, and his brethren the priests, before the tabernacle of the LORD in the high place that was at Gibeon,

Wesley's Notes

16:39 Zadok - The chief - priest at Gibeon, where the tabernacle and altar made by Moses still were, where also the ordinary sacrifices were offered, and the stated worship of God was performed, as the extraordinary worship was before the ark upon great occasions, as when God was consulted, which was to be done before the ark and by the high - priest, who was Abiathar.

Jamieson-Fausset-Brown Bible Commentary

39, 40. And Zadok . before the tabernacle . at Gibeon-While the above-mentioned officers under the superintendence of Abiathar, were appointed to officiate in Jerusalem, whither the ark had been brought, Zadok and the priests subordinate to him were stationed at Gibeon to perform the sacred service before the ancient tabernacle which still remained there.

Matthew Henry's Concise Commentary

16:37-43 The worship of God ought to be the work of every day. David put it into order. At Jerusalem, where the ark was, Asaph and his brethren were to minister before the ark continually, with songs of praise. No sacrifices were offered there, nor incense burnt, because the altars were not there; but David's prayers were directed as incense, and the lifting up of his hands as the evening sacrifice. So early did spiritual worship take place of ceremonial. Yet the ceremonial worship, being of Divine institution, must by no means be omitted; therefore at Gibeon, at the altars, the priests attended; for their work was to sacrifice and burn incense; and that they did continually, morning and evening, according to the law of Moses. As the ceremonies were types of the mediation of Christ, the observance of them was of great consequence. The attendance of his appointed ministers is right in itself, and encourages the people.