(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
1. The Shulchan Aruch (O.C. 260:1) writes that it is a mitzvah to cut one’s nails on Friday in order to honor Shabbos.
2. Harav Chaim Naeh zt”l (Ketzos Hashulchan 73 Badei Hashulchan 3) writes that there is only a mitzvah to cut one’s fingernails on Friday, since they are visible to others one should cut them in order to honor Shabbos. However, there is no obligation to cut one’s toenails on Friday. Yet, it should be noted that the Arizal would customarily cut his fingernails and toenails on Friday in honor of Shabbos (Shaar Hakavanos page 62).
3. The Aruch Hashulchan (260:6), Yesod V’Shoresh Haovodah (8:1) and Sifsei Cohen (cited in Taharas Hamayim tzadi 20) explain that it is preferable to cut one’s nails after midday (chatzos) on Friday. However, the Sefer Orchos Chaim (Spinka 285:4) cites those who rule that it is better to cut one’s nails before midday. It should be noted that the Ari Hakadosh would cut his nails after Mincha (which is also after midday) on Friday. (see Shaar Hakavanos ibid.)
4. The Mishna Berurah (260:6) writes that there is a custom not to cut one’s (fingernails) nails on Thursday. The reason being that nails begin to grow three days after being cut. Therefore if one cuts them on Thursday they will begin to grow on Shabbos. And causing them to begin growing on Shabbos reduces the level of honor for Shabbos, which was achieved by cutting them in the first place.
The Aruch Hashulchan (260:6) notes that there is no prohibition with causing them to grow on Thursday and therefore if one is unable to cut them on Friday he may do so on Thursday. In this instance it is better to cut them on Thursday than Wednesday (see Sefer Chut Shani Shabbos page 62).
[Harav Nissam Karelitz shlit”a adds that it goes without saying that if one’s nails grew long and are becoming bothersome one may cut them even during the week.]
5. Many poskim cite a custom not to cut one’s fingernails and toenails on the same day. Those who observe this custom should cut their toenails on Thursday and their fingernails on Friday (see Magen Avraham and Mishnah Berurah).
6. The Shulchan Aruch Harav (260:2) explains that the rational behind this custom is that it is “dangerous” to cut them both on the same day. The Yesod V’Shoresh Haovoda adds that it can also cause forgetfulness. This writer’s assumption is that these poskim were describing a spiritual danger and not a physical one.
7. In halacha the day follows the night. Based upon this concept the Gaon of Butchetch (Mili D’Chasidusa 57) writes that one may cut his fingernails during the day and toenails later that night since they were cut on two different hallachic days.
8. The Chida, however, writes that the Arizal would cut his fingernails and toenails on the same day and he was not particular with the previous halachos. He therefore writes that one may rule leniently as well.
9. Rav Yechial Safrin zt”l, known as the Komarna Rebbe, writes, “One may cut one’s fingernails and toenails on the same day since this was the custom of the Arizal. If one wishes to be stringent one should leave a baby toenail uncut.” (Shulchan Hatahor 260:3)
10. Harav Yehuda Hachasid writes in his will that one should not cut one’s hair or nails on Rosh Chodesh as this is considered spiritually dangerous. The Mishna Berurah adds that this is true even when Rosh Chodesh falls on a Friday. [There is a much larger discussion as to whether one must adhere to the will of Rav Yehuda Hachasid.]
11. We cited above (Halacha 4) that one should refrain from cutting one’s nails on Thursday as it causes them to grow on Shabbos. However, if Rosh Chodesh falls on Friday one may cut one’s nails on Thursday (Darkei Chaim V’Shalom 353 Az Nidberu 12:4).
12. Harav Yaakov Kaminetzky zt”l (Emes L’Yaakov Shulchan Aruch) maintains that if Thursday and Friday are Rosh Chodesh one may cut one’s hair on Friday. He begins by questioning the ruling of the Mishnah Berurah who warns against getting a haircut when Rosh Chodesh falls on a Friday since the performance of the mitzvah of preparing and honoring Shabbos should protect him from any danger that Harav Yehuda Hachasid warns against. He explains that the Magen Avraham and Mishnah Berurah feel that getting a haircut on Thursday is also a fulfillment of the mitzvah of honoring Shabbos. Therefore, if Rosh Chodesh falls on a Friday one should not get the haircut on that day since one can perform the mitzvah by getting a haircut on Thursday. However, if Rosh Chodesh is on Thursday and Friday one can not perform the mitzvah unless he gets a haircut on Rosh Chodesh. Therefore one should get his haircut on Friday and he can assume that the mitzvah of honoring Shabbos will protect him. The author assumes that this leniency would extend to cutting one’s nails as well, however, for normative halacha a Rav should be consulted.
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