Cutting nails on Thursday:
It is a Mitzvah to cut one’s [hand] nails every Erev Shabbos. If one is unable to wait until Friday, then one may cut his nails on previous days of the week. However, some are particular to not cut the nails on Thursday. [Practically, the widespread custom is to be careful as states this opinion. Thus, beginning from Wednesday night one should not cut his nails, until the start of Thursday night.]
It is customary to avoid cutting nails beginning from Wednesday night until the start of Thursday night.
Q&A on cutting nails on Thursday
If one will not have time to cut his nails on Friday may he be lenient to do so on Thursday?
If Yom Tov falls on Friday may one cut his nails on Thursday?
Yes. It is a Mitzvah to do so. [Thus, if Shavuos falls on Friday-Shabbos one may cut his nails on Thursday.]
May a Chasan and Kalah cut their nails on Thursday if their wedding is taking place that night?
If they did not cut the nails beforehand, and will be unable to cut the nails on Thursday night, they may cut their nails on Thursday.
May a woman get a manicure on Wednesday night/Thursday?
Seemingly, this is allowed as it does not involve cutting the nails off completely.
May one cut his toenails on Thursday?
It is unclear if those who avoid cutting nails on Thursday are only particular regarding the hand nails or also regarding the toenails.
May one cut the toenails after sunset on Thursday, prior to nightfall?
Seemingly, it is best to wait until nightfall to cut the nails, although in a time of need one may certainly do so.
 Admur 260:2
 The reason and the law by toenails: As the hand nails look repulsive when they are long. However, the toenails which are not visible are not considered a Mitzvah to be cut Erev Shabbos. [Ketzos Hashulchan 73 footnote 4] However, see Kaf Hachaim 250:17 who states that the Arizal would cut both his hand and toenails in honor of Shabbos.
 Admur ibid; Sefer Chassidim ibid
 The severity of this Mitzvah: Elya Raba 260:4 records a Midrash which states “There was once a Rabbi who passed away and appeared to his students in a dream with a blemish on his forehead. He told them this is due to his negligence in speaking in middle of the blessing of Meiyn Sheva and Kaddish, and due to him not being careful to cut his nails Erev Shabbos.” This story is also recorded in Sefer Chassidim ibid and is brought [without the conclusion of the nails] in Admur 268:17 and Taz 268:9
 May one initially cut the nails prior to Friday? According to all, if one can delay cutting his nails until Friday, it is a Mitzvah to do so, in order to emphasize that it is being cut in honor of Shabbos. [Admur ibid in first Stam statement; 1st custom in Admur ibid based on reasons of Kabballah (See next part of this footnote regarding the opinion of the 2nd custom); Elya Raba 260:5; Peri Megadim 260 M”Z 1; Piskeiy Teshuvos 260:5 footnote 54]
According to the custom of avoiding cutting nails on Thursday, may one do so prior to Thursday? It is implied that even according to the custom of being particular regarding Thursday, they allow one to cut the nails on Wednesday or another day of the week. See, however, Peri Megadim 260 M”Z 1 who brings the Elya Raba 260:5 who learns that this opinion holds that even before Thursday the nails are not to be cut, as then certainly nails will have grown by Shabbos. This understanding however requires further analysis as if so then what is the dispute between this opinion and the previous opinion. Accordingly, they both hold the nails are to only be cut on Erev Shabbos. This is besides the fact that this is not the simple implication of Admur, and does not fit in with the explanation of the Machatzis Hashekel [260-brought in the footnotes to follow] behind the stringency. Practically, Piskeiy Teshuvos ibid concludes that one is not to cut the nails even prior to Thursday unless it is a time of need, and so is our conclusion above, as at the very least this opinion would agree that there is greater respect in cutting the nails on Erev Shabbos, as writes Admur ibid in his opening statement.
 Taz 260:1; M”A 260; Rashal; Mateh Moshe 411
Other opinions: Some Poskim argue against this custom and conclude that all the cautions mentioned regarding nails are not required by the letter of the law, and one who is not particular in them, has not done any transgression. [Aruch Hashulchan 260:6]
 The reason: As the nails begin growing on the 3rd day after being cut and hence if they are cut on Thursday they will begin growing on Shabbos, thus some avoid cutting it on Thursday. [Admur ibid; Taz ibid] Now, although there is no prohibition involved if the nails begin growing back on Shabbos, nevertheless it is not respectful that the nails which one ridded himself of in honor of Shabbos begins growing back on Shabbos. [Machatziz Hashekel 260:1; M”B 260:6 in name of Elya Raba] The above applies only to nails however a haircut may be given on Thursday being that hair begins to grow back that same day. [Kuntrus Achron 260:1]
Regarding if also the toenails are not to be cut on Thursday according to this opinion: Perhaps one can say that this custom only applies to the hand nails being that they are visible. However, the toenails which are not visible, and thus there is no Mitzvah to cut them before Shabbos, then likewise there is no issue if they are cut on Thursday. Practically the M”B 260:6 rules that one is to cut the toe nails on Thursday. However, the wording of Admur seems to imply that they avoid cutting all nails on Thursday. Vetzaruch Iyun.
 Piskeiy Teshuvos 260:5 that Thursday night is considered Friday in this regard and the nails may be cut
 Aruch Hashulchan 260:6
 Aruch Hashulchan 260:6; Piskeiy Teshuvos 260:5
 Perhaps one can say that this custom only applies to the hand nails being that they are visible. However, the toenails which are not visible, and thus there is no Mitzvah to cut them before Shabbos, then likewise there is no issue if they are cut on Thursday. [Practically, so rules the M”B 260:6 that one is to cut the toe nails on Thursday, and so concludes Piskeiy Teshuvos 260 footnote 50 that the above adherence does not apply to toenails.] However, from the wording of Admur it seems to imply that one is to avoid cutting all nails on Thursday. Vetzaruch Iyun.