They held a feast before YHWH that day and celebrated. Then they designated Solomon, David's son, as king a second time; before YHWH they anointed him as ruler and Zadok as priest (1 Chronicles 29:22).
In 1 Kings 1:1-2:11, Solomon's accession to the throne after #David seemed less than certain.
The #Chronicler wanted to make sure no one was deceived: David fully intended #Solomon his son to rule after him as king.