Charles Ellicott Commentary Judges 21:4

"And it came to pass on the morrow, that the people rose early, and built there an altar, and offered burnt-offerings and peace-offerings." — Judges 21:4 (ASV)

Built there an altar. —We find David doing the same at the threshing-floor of Araunah (2 Samuel 24:25), and Solomon at Gibeon. Unless the entire tabernacle had, for the time, been removed to Bethel, there was no regular altar there. It has been suggested that in any case this altar must have been necessitated by the multitude of sacrifices required for the burnt offerings and the food of the people. (See Note on Judges 20:26.) Probably there is some other reason unknown to us.