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Cutting nails on Rosh Chodesh that falls on Erev Shabbos:
Normally, one may not cut their nails on Rosh Chodesh. This applies to both men and women. However, when Rosh Chodesh falls on Erev Shabbos some Poskim rule it is permitted to cut the nails in honor of Shabbos, and so is implied from Admur. Other Poskim, however, rule one may not cut the nails even in such a case. [Practically, one who cuts his nails on every Erev Shabbos may be lenient to do so also on Erev Shabbos Rosh Chodesh. Likewise, one with very long nails may be lenient to cut them.]
 Kneses Hagedola 260 writes “forbidden”, however the wording in Rav Yehuda Hachassid ibid and Beir Heiytiv is “One may not”; In Admur ibid the wording is simply that “Some places are accustomed” however this is referring to Erev Shabbos and not to every Rosh Chodesh.
 Rav Yehuda Hachassid 49; brought in: Olas Shabbos 260:1; Beir Heiytiv 260:2; Kneses Hagedola 260; omitted from Admur 260:1 and M”A 260:1; The Ketzos Hashulchan 73 footnote 4 questions as to why no mention of this is made by them. See however Shaar Hakolel in Nesiv Hachaim 48:3; Shivim Temarim 57 who explain Admur omitted it because on Erev Shabbos it is Mitzvah to cut the nails even when it coincides with Rosh Chodesh. However, on a non-Erev Shabbos Rosh Chodesh he would agree that one may not cut the nails. See next!
 The reason: As Rav Yehudah Hachasid warned that doing so involves danger. [Admur ibid; Beir Heiytiv ibid; See Likkutei Sichos 13:264]
The night of Rosh Chodesh: See Ashel Avraham Butchach Tinyana 417 that the night of Rosh Chodesh is more lenient in regards of cutting nails.
 Tehila Ledavid 260:1; Shaar Hakolel in Nesiv Hachaim 48:3; Shivim Temarim 57 as is the simple implication of the Kneses Hagedola ibid; Beir Heiytiv ibid; M”A and Admur ibid which omit the custom to avoid cutting nails when mentioning the custom to avoid cutting hair on Erev Shabbos Rosh Chodesh; opinions brought in footnotes on the Tzavah ibid; See Minchas Shabbos 72:66; Alef Hamagen 12; Maharam Brisk 2:99; Meishiv Halacha 2:4; Kitzur Hilchos Shabbos-Supplements.
The reason: If Rosh Chodesh falls on Erev Shabbos, the reason some have the custom not to cut their hair, despite the fact that this is usually a Mitzvah to do on Erev Shabbos, is because one is not obligated to cut his hair every Erev Shabbos. However being it is a Mitzvah to cut the nails on every Erev Shabbos, one is allowed to cut his nails even on Erev Shabbos Rosh Chodesh as one who fulfills a Mitzvah will know no harm. It thus seems that no such custom exists against cutting the nails on Erev Shabbos Rosh Chodesh and they should therefore be cut in honor of Shabbos. [Shivim Temarim ibid]
Other opinions: The Ketzos Hashulchan 72 footnote 4 leaves the opinion of Admur on this matter in question.
 M”B 260:7; Aruch Hashulchan 260:6; Kitzur SHU”A 72:14; Kaf Hachaim 260:12 allows only cutting the nails that stretch past the skin.
 Shivim Temarim ibid
 Kaf Hachaim ibid