The people rose early and built there an altar It is likely that they erected a new altar upon this present occasion, when such a multitude of sacrifices were to be offered by all the people of Israel, that the ordinary altar was not sufficient to receive them. Thus Solomon did when he dedicated the temple, 1 Kings 8:64. And, by the direction of God, it was frequently done in other places besides at the house of God.
Related Commentaries of Judges 21:4
Judges 21:4
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.