Thursday
4. The poskim say one should not cut his nails on Thursday (מ''ב סי' ר''ס סק''ו ). The poskim give different reasons for this, with practical differences between the reasons, as will be explained.
5. Resembles melacha. Some explain that since the nails begin to regrow on the third day from when they are cut, they will begin to regrow on Shabbos (ט''ז סק''א ), which looks somewhat like melacha. Although it is obviously not a true issur, as one did not do any melacha on Shabbos—just as one may water plants before Shabbos even though they will grow on Shabbos—nevertheless it still looks somewhat like melacha (שו''ת שבט הלוי ח''ו סי' כ''א אות ב' בביאור דעת הט''ז ). However, most Acharonim do not give this reason for this halacha.
6. Kevod Shabbos. Some explain it is a lack of kevod Shabbos. Since the nails begin regrowing on the third day, they begin to become less neat on Shabbos, and are thus not in the best possible shape on Shabbos (א''ר שם ). According to this, there is even more reason not to cut them on Wednesday, as they would be even less neat on Shabbos.
7. Contrary to kevod Shabbos. Some explain that it is because it is contrary to kevod Shabbos for the nails to begin growing that very day. However, if they start regrowing before Shabbos, it is not a problem. According to this, one can cut them on Wednesday since the main thing is not to have them start regrowing on Shabbos—but he will not have the advantage of cutting the nails in honor of Shabbos even though he is not entering Shabbos unkempt.
8. Wednesday night, Thursday night. For these halachos, Wednesday night has the status of Thursday, i.e., l’chatchila one should not cut his nails then. Similarly, Thursday night after tzeis hakochavim has the status of Friday; one may cut his nails then and it fulfills kevod Shabbos (קצוה''ש ).
Wednesday
9. Resembling melacha. According to the first reason (above, 5) that it looks somewhat like melacha, that is only when one cuts them on Thursday, causing them to start regrowing on Shabbos. However, if one cuts them on Wednesday, they begin regrowing on Friday; on Shabbos they merely continue to grow. This would be mutar.
10. Kevod Shabbos. According to the reason of kevod Shabbos (6), that they begin to become less neat on Shabbos, it is certainly worse to cut one’s nails on Wednesday, as mentioned. However, according to the reason that it is contrary to kevod Shabbos for them to begin regrowing on Shabbos (7), it is not a problem to cut them on Wednesday, as they do not begin regrowing on Shabbos, as mentioned.
