Q. In a case that Rosh Chodesh falls on Thursday and Friday, is one to get a haircut and cut nails on Wednesday? Due to Rebbe Yehuda Hachassid’s will.
A. Mishna Berura (260: 7) mentions that there are places where they do not shave or cut nails during Rosh Chodesh even when it coincides with Erev Shabbos, since it is the tzava’a and order of Rabi Yehuda Hechosid.
Most Poskim maintain that it applies to both days of Rosh Chodesh, however Yad Efraim is lenient on the first day of Rosh Chodesh and Eishel Avrohom (417) on the night of Rosh Chodesh. However, Kaf Hachaim (ibid. 12) opines that the proscription begins from the time of the molad, even when it is Erev Rosh Chodesh.
Maggen Avrohom and Eliahu Rabba, (260: 8) write that the tzava’a applies only to cutting the hair, other Poskim assert that it includes cutting nails also, as mentioned above, and when Rosh Chodesh is two days they should be cut on Wednesday (Elef Hamagen 12, and others).
Horav Shlomo Miller’s Shlit’a opinion is that when in need, if the nails are too long, they could be cut on Erev Shabbos Rosh Chodesh, if they were not cut before.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a