Charles Ellicott Commentary Numbers 6:5

"All the days of his vow of separation there shall no razor come upon his head: until the days be fulfilled, in which he separateth himself unto Jehovah, he shall be holy; he shall let the locks of the hair of his head grow long." — Numbers 6:5 (ASV)

There shall no razor come upon his head. —The hair was to be allowed to grow in all its luxuriance during the days of the Nazirite vow. The meaning may be illustrated by reference to Leviticus 25:4-5 and Leviticus 25:11, where it is prescribed that in the seventh year the vine was not to be pruned, and that the grapes of the undressed vine (literally, the grapes of the Nazirite) should not be gathered.