Cutting ones nails:
A. Cutting nails during the week, prior to Erev Shabbos?
Although it is a Mitzvah top cut the nails on Erev Shabbos as stated above nevertheless if one does not desire to wait until then, the question arises whether he may cut them during other days of the week. There are two customs recorded in this regard:
First Custom-Only cut on Erev Shabbos and Yom Tov: Some are cautious to avoid cutting their nails on any day other than Erev Shabbos or Erev Yom Tov. They thus cut their hand nails only on Friday and toe nails on Thursday. However on other days they do not cut their nails at all due to reasons known to them [based on Kabala].
Second Custom-Only avoid cutting on Thursday: Some [are accustomed to allow one to cut nails on other days of the week if he chooses not to wait until Friday, although they] are particular to not cut the nails on Thursday. [See Q&A] [Thus beginning from Wednesday night one should not cut his nails.]
B. Not to cut the toe and hand nails on same day:
Some are cautious to avoid cutting the hand and toe nails on the same day, due to worry of danger. [However one may cut one during the day and the other that night. Some also allow to cut one of them at night and the other the next day however others rule this is not to be done, and so seems to be the opinion of Admur.]
C. The order of cutting the nails:
Which hand is one to begin with? One first cuts the nails of his left hand and then the right. 
Skipping a nail in between each cut: It is proper to initially be careful not to cut the [hand] nails one finger after the other, in the set order of the hand. Rather one is to skip the finger that is adjacent to each finger that is cut, and cut the finger which is adjacent to the skipped finger. Hence one is to begin with his left hand cutting first the index finger closest to the pinky and skipping one finger each time. One then cuts the nails of the right hand beginning with the index finger closest to the thumb, skipping one finger each time. Hence the number order of the cutting beginning from thumb to pinky [thumb is #1 pinky is #5] is for the left hand 4,2,5,3,1. By the right hand it is 2,4,1,3,5.
D. What is one to do with the nails after they are cut?
Nails contain a certain impurity which can be lethal for the fetus of a pregnant woman, and cause miscarriage, if she steps on it. For this reason it is necessary to discard one’s nails properly to assure that a pregnant woman will not come to step on it. There are three different ways of discarding the nails relative to three types of characteristics.
- Chassid: A Chassid burns his nails. [The Rebbe Rashab would mix some wood together with the nails before burning it.] [See Q&A]
- Righteous person: A righteous person buries his nails.
- Rasha: A Rasha discards his nails on the floor.
Throwing one’s nails in a Beis Hamidrash: It is permitted for one to throw his nails in a Beis Midrash and places of the like in which women are not accustomed to enter.
Nails which have been swept to a different area: The nails are only considered a danger while in the original area that they were thrown in after they were cut. If however they are swept to another area then they are no longer a danger.
E. Washing ones hands after nail cutting:
One is to wash his hands [immediately] after cutting his nails. [This applies to both one’s hand and toe nails, and applies whether one cut his nails with a nail cutter or with his teeth.]
The danger involved in not washing the hands: If one does not wash his hands after cutting his nails, then he will have inner fear for one day. In addition if he is a Torah scholar, he will forget his learning, while if he is a layman, he will lose his mind.
How to wash: One must use water to clean his hands. It does not suffice to clean his hands in other ways [such as to rub them on something. The reason for this is because an impure spirit resides on ones hands after these actions are done. Thus one needs to wash his hands even if he does not plan to pray or learn Torah afterwards.] However one does not need to pour the water on his hands three times as is required when washing upon awakening. [The entire hand is to be washed until ones wrist, or at least until ones knuckles. The above washing is only in order to remove impurity, and does not prevent one from studying Torah or praying beforehand.]
Summary and practical guideline:
It is a Mitzvah to cut one’s finger nails on Erev Shabbos. When cutting during other days of the week one is to avoid cutting the nails on [Wednesday night through] Thursday. One is to avoid cutting the hand and toe nails the same day.
One is to begin cutting the nails of his left hand starting with the index finger closest to the pinky. One skips a finger between each nail. On his right hand one first cuts the index finger closest to his thumb and then skips a finger in between each nail. One is to discard the nails properly, such as in the toilet or through burning them. One is to wash his hands after cutting the nails.
When on Erev Shabbos are the nails to be cut?
The nails are to be cut prior to bathing for Shabbos or going to Mikveh. Regarding when they are to be cut prior to Mikveh or bathing there are various opinions mentioned: Before Midday, [after midday before Mincha], after Mincha, before Shnayim Mikra, after Shnayim Mikra but before Mikveh. We have not accepted any final ruling. [Practically many have the custom to cut the nails prior to Shnayim Mikra.]
May one cut his nail when Rosh Chodesh falls on Erev Shabbos?
May one cut his nails on Erev Shabbos Chol Hamoed, or Erev Yom Tov Achron?
If one cut his nails on Erev Yom Tov: One who cut his nails on Erev Yom Tov [or in close proximity to Erev Yom Tov to the point there was nothing to cut before Yom Tov] is permitted to cut his nails during Chol Hamoed.
If one did not cut them Erev Yom Tov: Some Poskim rule it is forbidden to cut nails on Erev Shabbos, or Erev Yom Tov of the last days, if he did not cut them on Erev Yom Tov of the first days. So is also implied to be the opinion of Admur. Others however rule it is permitted to cut nails on Erev Shabbos [or Erev Yom Tov] if one is accustomed to do so on every Erev Shabbos throughout the year. According to all it is forbidden to cut the nails on Erev Shabbos if he is not accustomed to always do so.
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.
May a Chasan and Kallah 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 be cut on Thursday.
May one cut his toe nails at night and his hand nails the next day?
Some Poskim rule this is not to be done. Others rule it is allowed.
If one did not cut his toe nails prior to Erev Shabbos, may he cut them on Erev Shabbos together with his hand nails?
May one cut his toe nails on Thursday?
It is unclear if those which avoid cutting nails on Thursday are only particular regarding the hand nails.
Must one skip a nail also when cutting the toe nails?
No. This cautiousness is only relevant to the hand nails.
What is one to do if a nail dropped and became lost?
He is to sweep the dirt of that area into another room.
May one throw the nails into the sink or toilet rather than burn or bury them?
Yes. Throwing the nails into the sink or toilet is similar to burying them. Nevertheless one who wishes to follow the practice of a Chassid is to specifically burn them rather then throw them in the toilet.
May one learn Torah while cutting his nails?
Must one wash his hands after cutting another person’s nails, such as one’s children?
No. One is only obligated to wash his hands after cutting his own nails.
Must one wash their hands if another person cut his nails? 
Sparks of Kabala-Meaning behind nails:
The nails are rooted in a very high spiritual source. Prior to the sin of eating from the tree of knowledge Adam was completely covered by nails which protected him from evil. The sin then caused the nails to leave and only remain on the tips of the fingers, in which there also resides the impurities of the sin. Due to this the nails must be removed, and doing so brings peace to the world. However they may not be belittled by throwing them away as they derive from a high source and on them is dependant the fixing of the world.
It is also for this reason that they are dangerous for women. This is measure for measure as the woman caused Adam to lose his sublime revelation contained within the nails, therefore the nails cause her danger.
 As the hand nails look repulsive when they are long. However the toe nails which are not visible are not considered a Mitzvah to be cut Erev Shabbos. [Ketzos Hashulchan 73 footnote 4] However see Kaf Hachayim 250/17 which states that the Arizal would cut both his hand and toe nails in honor of Shabbos.
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”.
Although it is a Mitzvah to cut ones nails specifically on Erev Shabbos, as stated above, the question here is may one cut them on other days if he so chooses.
 Being that the hand and toe nails are not to be cut the same day, as will be explained, therefore they cut the toe nails on Thursday rather than Friday. Thus this custom seemingly does not hold of the custom to avoid cutting nails on Thursday. [Kitzur Hilchos Shabbos 260] Perhaps however one can say that this custom of avoiding cutting the nails on Thursday only applies to the hand nails being that they are visible. However the toe nails 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 this is the custom that one is to follow.
 The Aruch Hashulchan [260/6] argues against this custom. Likewise he writes that all these cautions mentioned by nails are not required by the letter of the law, and one who is not particular in them, has not done any transgression.
 It is implied that this opinion is only particular regarding Thursday, however they allow one to cut the nails on Wednesday or another day of the week. See however Peri Megadim 260 M”Z 1 which brings the Elyah Raba which learns that this opinion holds that even before Thursday the nails are not to be cut, as then certainly one will have grown nails 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.
 However according to all if one is able to delay cutting his nails on Friday, it is a Mitzvah to do so in honor of Shabbos, as explained above.
 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. [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] 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 toe nails 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 toe nails 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.
 The Arizal was not careful to avoid cutting the nails on the same day. Practically the Mateh Efraim rules that one is to be stringent in this matter. [Ketzos Hashulchan 73 footnote 5]
 However women which are preparing for Mikveh may do so being that Hashem guards those which are observing a Mitzvah.
 Ashel Avraham Butshatch, brought in Ketzos Hashulchan 73 footnote 4.
 See Ashel Avraham ibid which suggests at first that the night has no relation to the day in this regard. However later he mentions that the night goes after the next day and not the previous day. So rules also Ashel Avraham Tinyana in 260 that the night goes after the day and one is to beware from cutting on Thursday night as well as on Thursday. So is also implied from Admur 260/2 that does not mention to cut the toe nails on Thursday night rather than Thursday. However see Piskeiy Teshuvos 260 footnote 84 for opinions that rule that the night does not follow the previous day or the following day.
 Although in Admur there seems to be a slight discrepancy [see below] regarding which hand one is to begin with, the Ketzos Hashulchan [73/2] rules to begin with the left hand, and so rules Kalbo [brought in Aruch Hashulchan 260/6], Eliyahu Raba and Peri Megadim based on Rama which mentions the left hand first.
Other Opinions–Begin with right: The Aruch Hashulchan himself [ibid] rules that one is to begin with the right hand and then the left, and so rules Ashel Avraham [Butchatcher] 260, based on the Talmudic ruling that the right is always to be given precedence over the left.
Discrepancy in Admur: Admur began the law of cutting nails with the left hand and then the right, and then concluded with the right and then the left. This is especially puzzling being the Rama concluded also first with the left and then with the right, and hence for what reason did Admur change from the order of the Rama. Some desire to explain that Admur in truth holds there is no preference as to which hand one cuts first. Vetzrauch Iyun.
 Admur 260/3 records a difference of opinion regarding this matter: Some are cautious to skip a nail when cutting their nails. Others [Arizal/Tashbatz brought in Taz 260/2 and M”A 260/1] however belittle this custom. Admur concludes: Nevertheless it is good for one to initially be cautious in this matter. [ibid as rules M”A 260/1]
 However by the toe nails there is no need to be careful to skip a nail in between. [Ketzos Hashulchan 73 footnote 7]
 As they believe that doing so brings one towards forgetfulness, causes one to bury his children, and brings one to poverty. [ibid]
 In order so they are destroyed from the face of the earth and there is no longer any possibility for them to become uncovered and cause potential damage to a fetus. One who does so is called a Chassid being that burning part of one’s body, even one’s nails, is damaging for oneself, hence he is called a Chassid as he overrides his own health in order to fully protect his friend. [Machatzis Hashekel 260/1]
 Hayom Yom 16 Iyar
 This is however of less righteousness then is a Chassid, as it is possible for the nails to resurface.
 He is considered a Rasha as he is causing a potential hazard if a pregnant woman were to walk on it.
 It requires further analyses with regards to the definition of “area”? Bircheiy Yosef 260/6 rules like the Perisha that as long as the nail has moved from the exact area it fell it is no longer considered dangerous, even if it remains in the same room. So rules also Aruch Hashulchan 260/6. The Elya Raba rules that “area” means a room, thus only when the nail has been swept to another room is it considered to have switched areas. [brought in M”B 260/6] However if placed into another area within the same room the Elya Raba questions whether this suffices to remove the danger. [see Peri Megadim A”A 260]
 The Aruch Hashulchan 260/6 notes that if one cut another item with the nail cutter after the nails were cut, the danger is no longer applicable.
 Kama 4/18
 Magen Avraham and Peri Megadim 4/18. Vetzaruch Iyun as Admur omitted this.
 Migdal Oaz [Yaavetz]; Perisha 241; Ketzos Hashulchan 2/11 footnote 27; Suppliments p. 82
 Ketzos Hashulchan 2/11
 Mahadurah Kama 4/18
 Kama 4/19
 Meaning that he will come to sin, as one does not sin unless a spirit of insanity enters him. [Kaf Hachayim 4/89]
 Ketzos Hashulchan 2/11. See also Admur 97/3 and Peri Megadim Ashel Avraham 227/2. However see Kaf Hachayim 4/90 which brings an opinion which holds that not all of the following actions bring an impure spirit. Admur however seems to hold that they all do, being that he rules that only washing with water helps.
 Ketzos Hashulchan 2/11
 Ketzos Hashulchan 2/11
 See Admur 7/2; 613/4; 92/6; Peri Megadim 227 A.A. 2 explicitly allows saying a blessing before washing hands from impurity; Mahrsham 4/148 brought in Shearim Hametzuyanim Behalacha 2/5 regarding learning while cutting nails.
In 7/2 Admur rules that one who used the bathroom may learn Torah and say blessings unless he touched his Erva or feces. Now in 4/18 Admur explicitly rules that one who uses the bathroom must wash hands. Hence we clearly see a difference between washing hands from filth and washing form impurity, in which case of the latter it is not an impediment for Torah learning.
The reason: The impurity which comes as a result of the above actions is not the same type of impurity as that which resides when one awakens from sleeping at night, and thus it does not hold the restriction mentioned prior to washing hands in the morning.
 Shaareiy Halacha Uminhag 130
 Kaf Hachayim 260/1 as was the custom of the Arizal; Mateh Efrayim 625/13; Shlah ibid
To note however of the custom of the Rebbe Maharash which would cut the nails after Mikveh. His reasoning was because at this time the nails are softer. [Story heard from Rav Zalmon Shimon Dworkin]
 Aruch Hashulchan 260/5-6
 The Magen Avraham brings in the name of the Shlah [p. 138] that one should cut the nails on Erev Shabbos prior to saying Shnayim Mikra [which is then followed by Mikveh]. However in Peri Eitz Chaim it states to cut them after Shnayim Mikra, prior to immersing in the Mikveh. Elsewhere it is brought in the name of the Arizal to cut them prior to midday. This however contradicts that which is brought in Shaar Hakavanos that the Arizal would cut his nails after Mincha.
 Shaar Hakolel in Nesiv Hachayim 48/3; Opinions brought in footnotes on Tzavah Rav Yehuda Hachassid]. see Kitzur Halachos Shabbos-Supplements; Ketzos Hashulchan 72 footnote 4 leaves this matter as a Tzaruch Iyun.
Background of ruling: If Rosh Chodesh falls on Erev Shabbos, although some have the custom not to cut their hair, despite the fact that this is usually a mitzvah to do on Erev Shabbos, nevertheless regarding cutting ones nails it seems that no such custom exists and they should therefore be cut in honor of Shabbos. This is based on the law written in Admur regarding Rosh Chodesh which falls on Shabbos, in which Admur only writes that cutting hair is refrained while cutting nails is omitted. To note that this omission of cutting nails is also within the Magen Avraham and Kneses Hagedola which is the source of the ruling regarding haircuts.
 M”B 260/7; Aruch Hashulchan 260/6; Kaf Hachayim 260/12 allows only cutting the nails that stretch past the skin.
 M”A 532/1; Chayeh Adam 109/3; M”B 532/2; Kitzur SH”A 104/12; Chol Hamoed Kehilchasa 4/5; Kaf Hachaim 532/7
Other Opinions: The Elya Raba 532/1 rules one may only remove the nails with a knife and not with scissors.
 Aruch Hashulchan 532/2
 The reason for this allowance is because even regarding a haircut there are Poskim which allow it to be done on Chol Hamoed if one cut his hair on Erev Yom Tov. Thus regarding cutting nails which is itself disputed whether it is at all forbidden during Chol Hamoed, one may certainly be lenient if he cut his nails on Erev Yom Tov. [M”A 532/1; P”M 532 A”A 1]
 See Shaareiy Teshuvah 468/1; Kaf Hachaim 532/5
 Shvus Yaakov 1/17; Koveitz Mibeis Levi 1/47 that so is the custom; Piskeiy Teshuvos 532/1
 468/6 from the fact he forbids cutting the nails on Erev Pesach past Chatzos even though it is Erev Yom Tov. See Shvus Yaakov ibid. It is thus a wonderment that Chol Hamoed Kihilchasa 4/6 rules plainly that it is allowed despite this clear ruling of Admur.
 Nachalas Shiva 2/57; Shulchan Gavoa 532/2; Chol Hamoed Kihilchasa 4/6; Kaf Hachaim 532/5 concludes one who is lenient has upon whom to rely.
 Chol Hamoed Kihilchasa 4/6
 As it is permitted to cut nails for the sake of a Mitzvah and cutting them on Erev Shabbos in honor of Shabbos is a Mitzvah. [ibid]
 Aruch Hashulchan 260/6
 Aruch Hashulchan 260/6
 Ashel Avraham Butshatch, brought in Ketzos Hashulchan 73 footnote 4 suggests at first that the night has no relation to the day in this regard. However later he mentions that the night goes after the next day and not the previous day. So rules also Ashel Avraham Tinyana in 260 that the night goes after the day and one is to beware from cutting on Thursday night as well as on Thursday. So is also implied from Admur 260/2 that does not mention to cut the toe nails on Thursday night rather than Thursday. However see Piskeiy Teshuvos 260 footnote 84 for opinions that rule that the night does not follow the previous day or the following day.
 Mateh Efrayim 625/14-Alef Lamateh 21
 Perhaps one can say that this custom only applies to the hand nails being that they are visible. However the toe nails 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.
 Ketzos Hashulchan 73 footnote 7
 M”B 260/6
 As once it has switched rooms, the nail is no longer of danger.
 Beir Moshe 6/133-12
 Mahrsham 4/148 brought in Shearim Hametzuyanim Behalacha 2/5
 Ashel Avraham Buchacher Mahdurah Tinyana 4/18
 So is also implied from Admur 4/18 “Cutting his nails” and not plainly “cutting nails”.
 Ashel Avraham Buchacher Mahdurah Tinyana 4/18
 Shalah Hakadosh p. 138 in name of Tolas Yaakov