Cutting hair and nails on Rosh Chodesh:
One may not cut the hair of his head [or beard] or his nails on Rosh Chodesh. This applies to both men and women. [This applies to both days of Rosh Chodesh. This applies to both the day and night of Rosh Chodesh. Some Poskim rule one should not cut the hair or nails starting from the time of the Molad, until after Rosh Chodesh.]
When Rosh Chodesh falls on Erev Shabbos: Even when Rosh Chodesh falls on Erev Shabbos, some communities are accustomed not to cut their hair in honor of Shabbos due to the above warning of danger of Rav Yehuda Hachassid. [See Q&A regarding cutting nails in such an instance!].
May one cut his nail when Rosh Chodesh falls on Erev Shabbos?
Some Poskim rule it is permitted to cut the nails on Erev Shabbos Rosh Chodesh, and so is the implied opinion of Admur. Other Poskim, however, rule one may not cut the nails on Rosh Chodesh even when it falls on Erev Shabbos. [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. However, some Poskim conclude that initially it is proper to cut the nails on Thursday, or Wednesday if Rosh Chodesh falls on Thuursday-Friday, and hence avoid entering the dispute.]
If Rosh Chodesh falls on the day of one’s Mikveh night should the nails be cut prior to Rosh Chodesh?
May a woman shave her legs and the like on Rosh Chodesh?
May a man trim his mustache on Rosh Chodesh?
Rav Yehuda Hachassid adds “Zakan” Seemingly one may not trim the mustache on Chol Hamoed. Vetzaruch Iyun.
If one began cutting his nails or hair and then remembered that it is Rosh Chodesh may he finish cutting them?
If one began cutting his nails [or hair] on Rosh Chodesh and then realized his mistake, he may continue to cut the remainder, and Hashem will protect him of his mistake. This applies even if he began cutting only a small part of the nail.
Is a boy who was born on Rosh Chodesh to have his Upsherinish haircut take place on his third birthday on Rosh Chodesh?
The hair is not to be cut on Rosh Chodesh but rather immediately after Rosh Chodesh.
 Tzavas Rav Yehudah Hachasid 48 [hair] 49 [nails] brought in: Olas Shabbos 260:1; M”A 260:1 and Admur 260:1 regarding [only] hair; Beir Heiytiv 260:2 [regarding hair and nails]; Kneses Hagedola 260 regarding hair; Elya Raba 260:8 regarding hair
 Kneses Hagedola ibid 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.
 See Q&A regarding women shaving.
 Tzavas Rav Yehuda Hachassid ibid
 Rav Yehuda Hachassid 49; Beir Heiytiv ibid; Kneses Hagedola ibid; omitted from Admur ibid and M”A ibid; The Ketzos Hashulchan 73 footnote 4 questions as to why no mention of this is made by the M”A or Admur in 260:1. 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!
Other customs: Some Rabbanim are accustomed to rule that there is no need to be stringent to not cut nails on Rosh Chdoesh, and they only suspect for the words of Rav yehuda Hachassid regarding cutting hair. [Yosef Ometz 37 in name of Rabbanim]
 The reason: As Rav Yehudah Hachasid warned that doing so involves danger. [Admur ibid; Olas Shabbos 260:1; Beir Heiytiv ibid; See Likkutei Sichos 13:264]
 Setimas Kol Haposkim; Piskeiy Teshuvos 260:10 footnote 95
Other opinions: Some Poskim rule it is permitted to cut nails on the first day of Rosh Chodesh. [Yad Efraim in Kuntrus Achron of Sefer Chassidim]
 Setimas Kol Haposkim; Piskeiy Teshuvos 260:10 footnote 95
Other opinions: Some Poskim rule it is permitted to cut nails on the night of Rosh Chodesh. [Ashel Avraham Butchach Tinyana 417; Mili Dechassidusa 57]
 Yifei Laleiv 260:5; Kaf Hachaim 260:12
 Admur 260:1; Kneses Hagedola 260; M”A 260; Beir Heiytiv 260:2
Other Opinions: Some Poskim rule it is permitted to cut one’s hair in honor of Shabbos even if Erev Shabbos is Rosh Chodesh, as one who fulfills a Mitzvah will know no harm. [Tehila Ledavid 260:1]
 See Piskeiy Teshuvos 260:10
 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, Elya Raba ibid, M”A ibid, and Admur ibid who all omit the custom to avoid cutting nails when mentioning the custom to avoid cutting hair on Erev Shabbos Rosh Chodesh [However, see Ketzos Hashulchan 72 footnote 4 who leaves the opinion of Admur on this matter in question.]; 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: The reason some have the custom not to cut their hair on Rosh Chodesh even when it falls on Erev Shabbos, despite the fact that it is a Mitzvah to get a haircut 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 [see wording of Admur ibid], 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]
 Derech Hachaim Dinei Erev Shabbos; Ashel Avraham Butchach Tinyana 417; Mili Dechassidusa 57; Kitzur SHU”A 72:14; Aruch Hashulchan 260:6; M”B 260:7 “There are communities who do not cut their hair or nails on Rosh Chodesh even when it falls on Shabbos”; Likkutei Maharich “It is difficult to do an action against the great Poskim”; Kaf Hachaim 260:12 allows only cutting the nails that stretch past the skin; Piskeiy Teshuvos 260:10 “Initially one is certainly to suspect for the ruling of these Poskim”; See Shaareiy Teshuvah 260:1 in name of Machazik Bracha 260; Ketzos Hashulchan 72 footnote 4 leaves this matter in question
 Shivim Temarim ibid; Orchos Chaim Spinka 260:3 in name of Alef Hamagen 12 and Piskeiy Teshuvos ibid that if one forgot to cut them on Thursday/Wednesday, one may cut them on Erev Shabbos in such a case even if he only cuts his nails every other Shabbos
 Mili Dechassidusa ibid; Kaf Hachaim ibid; Piskeiy Teshuvos ibid
 Shaareiy Teshuvah 260:1 in name of Machazik Bracha 260; Ashel Avraham Butchach Tinyana 417; Mili Dechassidusa 57; Darkei Chaim Veshalom 394; Az Nidbaru 12:4; Piskeiy Teshuvos 260:10
 Tehila Ledavid 260:1
 The reason: As one who does a Mitzvah will see no harm.
 Tzavas Rav Yehudah Hachasid ibid: “A person is not to shave his head or beard on Rosh Chodesh”
 Ashel Avraham Butchach 417 Tinyana; brought in Ketzos Hashulchan 73 footnote 4
 Igros Kodesh 18:191; 22:372 [printed in Shulchan Menachem 2:202]; Kapos Temarim 14 [on the Sefer Shivim Temarim]