2,914. When Is It a Mitzvah to Shave?

Hilchos Nezirus 7:14

Let’s say that there are two nazirs, one who committed to nezirus for 30 days and one who committed to nezirus for 100 days. The two of them find a corpse that would be a mitzvah to bury. The one whose nezirus is 30 days is the one who should allow himself to be rendered unclean. If one nazir pledged nezirus for a set duration and the other will be a nazir forever, the one who accepted nezirus for a limited time is the one who should allow himself to be rendered unclean. This is because an eternal nazir has a higher degree of sanctity.

Hilchos Nezirus 7:15

Shaving for purposes of a mitzvah means that if a nazir contracts tzaraas and is healed during his nezirus, he is to shave off all his hair. This is because shaving fulfills a Torah obligation, since Leviticus 14:8 says that a metzora (“leper”) “shall shave all of his hair.” When there’s a conflict between a positive and a negative mitzvah, we try to fulfill both. If that’s not possible, the positive mitzvah takes priority over the negative. However, if a nazir shaves during his nezirus period, he violates both a negative and a positive mitzvah: Numbers 6:5 says that a nazir’s hair “is holy. He must let the growth of his head’s hair grow.” A positive mitzvah doesn’t “outrank” a negative mitzvah plus a positive mitzvah. So why does the positive mitzvah for a metzora to shave outrank his nezirus? Because in this case the nazir is already unclean from the tzaraas and the days when he is considered unclean don’t count towards nezirus, as has already been discussed, with the result that his hair isn’t considered holy for the duration. Therefore, the positive mitzvah (to treat a nazir’s hair as holy) is suspended, leaving just the negative mitzvah of “a razor shall not pass over his head.” Therefore, the positive mitzvah to shave a metzora can take precedence over it.