C. F. Keil and Franz Delitzsch on 2 Chronicles 8:12

2Ch 8:12 The sacrificial service in the new temple. Cf. 1Ki 9:25, where it is merely briefly recorded that Solomon offered sacrifices three times a year on the altar built by him to the Lord. In our verses we have a detailed account of it. אז, at that time, scil. when the temple building had been finished and the temple dedicated (cf. 2Ch 8:1), Solomon offered burnt-offerings upon the altar which he had built before the porch of the temple. He no longer now sacrifices upon the altar of the tabernacle at Gibeon, as in the beginning of his reign (2Ch 1:3.).