Rashi on Numbers 6:18/רש"י על במדבר ו׳:י״ח

1וגלח הנזיר פתח אהל מועד AND THE NAZARITE SHALL CLIP [THE HAIR OF THE HEAD OF HIS NAZARITEHOOD] AT THE ENTRANCE OF THE APPOINTED TENT — One might think that he must cut his hair in the forecourt (for it states at the entrance of the appointed tent)! Surely, this would be treating the forecourt in a contemptuous manner! But the meaning is: the Nazarite shall cut his hair after the feast-offerings had been slaughtered of which Scripture writes, (Leviticus 3:2) “and he shall slaughter it פתח אהל מועד” (cf. Sifrei Bamidbar 35; Nazir 45a).

2אשר תחת זבח השלמים [AND HE SHALL TAKE THE HAIR … AND PUT IT IN THE FIRE] WHICH IS UNDER THE SACRIFICE OF THE FEAST-OFFERINGS — i.e., under the pot in which he boils it (the שלמים); because the feast offerings of the Nazarite were boiled in the forecourt since the priest had to take “the shoulder” after it had been boiled (cf. vv. 19 and 20) and to wave it before the Lord (cf. Nazir 45b).