Question: [Motzei Shabbos, 6th Kisleiv, 5781]
May one get a haircut on Motzei Shabbos? My son is not home from yeshiva that often and because of the short Friday we have not had time to give him a haircut on Erev Shabbos. May I give him a haircut on Motzei Shabbos before he returns to yeshiva? He has very long hair already and I don’t know if we will have time to do so on Erev Shabbos as we would usually try to do.
Yes, you may give him a haircut on Motzei Shabbos.
Explanation: There is no unique issue with getting a haircut on Motzei Shabbos or at nighttime other than the fact that initially one is to schedule a haircut for Erev Shabbos, as so is the Mitzvah Min Hamuvchar. It is especially befitting not to initially schedule it for Motzei Shabbos, being that it shames the Shabbos showing in that one doesn’t care to look presentable for it. Nonetheless if one has a valid reason for cutting it prior to Erev Shabbos then he is not required to wait until Erev Shabbos, and may do so on Thursday, and if necessary may do so even earlier, including on Motzei Shabbos, if needed. Certainly, in your case it is a valid reason to cut it on Motzei Shabbos, in order so a) by the next Shabbos he will for certain have normal hair as well as so b) it is not a Chatzitza for his Tefillin and so they can better fit onto his head and c) so he can look presentable as a Yeshiva Bochur
Sources: See Admur 260:1 that it is a Mitzvah Min Hamuvchar to cut the hair specifically on Erev Shabbos and not beforehand. Chochmas Shlomo 493 regarding Motzei Shabbos; However, if one has a valid reason for requiring a haircut during the week then he is not required to wait until Erev Shabbos. [Mahari Tzemach brought in Birkei Yosef 232:3; Kaf Hachaim 232:15] Regarding getting a haircut at night see: Pekudas Eliezer 493