Parashas Naso · Passuk 6:05
Throughout the term of their vow as nazirite, no razor shall touch their head; it shall remain consecrated until the completion of their term as nazirite of GOD, the hair of their head being left to grow untrimmed.
Chabad.org — English translation + Rashi
Source: Nazir 39a.
English: “All the days of his nazirite vow” — a nazir whose days have been completed and who then shaves is liable.
In context: The prohibition on cutting hair, like the wine prohibition, extends until the formal conclusion of the naziriteship. Many units elaborate the details of the hair restriction.
Custom
Chazal extend the razor prohibition until the nazir’s concluding rites are performed.