Charles Ellicott Commentary Numbers 6:18

"And the Nazirite shall shave the head of his separation at the door of the tent of meeting, and shall take the hair of the head of his separation, and put it on the fire which is under the sacrifice of peace-offerings." — Numbers 6:18 (ASV)

And the Nazarita shall shave ... — The consignment of his hair to the sacrificial fire formed a solemn and suitable termination of the days of his separation to the Lord during the continuance of his Nazirite vow.