Rashi on Leviticus 21:10/רש"י על ויקרא כ״א:י׳

1לא יפרע [THE HAIR OF HIS HEAD] SHALL HE NOT DISARRANGE — i.e. he shall not let his hair grow wild on account of ritual mourning for the death of a near relative (Sifra). And what is called "letting the hair grow wild”? Leaving it uncut for more than 30 days (Sanhedrin 22b).