At that time when David saw that the LORD had answered him in the threshingfloor of Ornan the Jebusite, then he sacrificed there.

Wesley's Notes

21:28 Sacrificed - When he perceived that his sacrifice was acceptable to God, he proceeded to offer more sacrifices in that place.

Jamieson-Fausset-Brown Bible Commentary

28. when David saw that the Lord had answered him ., he sacrificed there-or, "he continued to sacrifice there." Perceiving his sacrifice was acceptable, he proceeded to make additional offerings there, and seek favor by prayer and expiatory rites; for the dread of the menacing angel destroying Jerusalem while he was absent in the center of worship at Gibeon, especially reverence for the Divine Being, led him to continue his adorations in that place which God (2Ch 3:1) had hallowed by the tokens of His presence and gracious acceptance.

Matthew Henry's Concise Commentary

21:1-30 David's numbering the people. - No mention is made in this book of David's sin in the matter of Uriah, neither of the troubles that followed it: they had no needful connexion with the subjects here noted. But David's sin, in numbering the people, is related: in the atonement made for that sin, there was notice of the place on which the temple should be built. The command to David to build an altar, was a blessed token of reconciliation. God testified his acceptance of David's offerings on this altar. Thus Christ was made sin, and a curse for us; it pleased the Lord to bruise him, that through him, God might be to us, not a consuming Fire, but a reconciled God. It is good to continue attendance on those ordinances in which we have experienced the tokens of God's presence, and have found that he is with us of a truth. Here God graciously met me, therefore I will still expect to meet him.