Question: Is there any reason not to take a haircut on Asara B’teves?

It is permissible to take a haircut both on the night preceding and on the day of one of the minor fasts (i.e. Asara B’teves, Tzom Gedalya, and Ta’anis Esther)1)Shulchan Aruch Orach Chaim 550:2.  There is a little known stringency however regarding this matter, brought down in the Mishnah Brurah2)Mishnah Brurah 550:6 and advocated only for people who are on the level of “ba’al nefesh”.

Regarding the fast of the 17th of Tammuz (though it is a minor fast) the law is different for Ashkenazim since it begins the period of the three weeks, and we don’t take haircuts during that period. Rav Moshe Feinstein says that in a time of need a person can take a haircut on the night of the 17th of Tammuz. It goes without saying that on Tisha B’av one can not take a haircut as it is a day of mourning and mourners can’t take haircuts.

References   [ + ]

1. Shulchan Aruch Orach Chaim 550:2
2. Mishnah Brurah 550:6