Albert Barnes Commentary 2 Samuel 15:7

"And it came to pass at the end of forty years, that Absalom said unto the king, I pray thee, let me go and pay my vow, which I have vowed unto Jehovah, in Hebron." — 2 Samuel 15:7 (ASV)

Forty years - An obvious clerical error for "four years," though a very ancient one, which may date from Absalom’s return from Geshur, his reconciliation with David, or the commencement of the criminal schemes to which 2 Samuel 15:1 refers.

Hebron - This, as the former capital of David’s kingdom and Absalom’s birthplace, was well chosen. It was a natural center, probably had many inhabitants discontented with the transfer of the government to Jerusalem, and contained many of the friends of Absalom’s youth. As the place of his birth (compare to 1 Samuel 20:6), it afforded a plausible pretext for holding the great sacrificial feast there (the serving the Lord, 2 Samuel 15:8), which Absalom pretended to have vowed to hold for the glory of God.