There is indeed an ancient custom of not cutting nails sequentially. The earliest mention seems to be in a version of Masechet Kallah quoted by the Machzor Vitry, written by Rabbi Simchah ben Shmuel of Vitry (who passed away in 1105, the same year as his teacher, Rabbi Shlomo Yitzchaki (Rashi)).1 There we are told that one should be careful not to cut his nails in order, since it can bring forgetfulness, poverty, and premature death of one’s children.2

Sounds mysterious? Well, to add to the mystery, here is a cryptic Hebrew phrase related to this custom:3

קשי״א בל״א תירו״ץ

This literally translates as “question without answer,” which some take as an indicator that the deeper reasons behind this custom are not readily apparent.4

This phrase is actually a mnemonic that tells us the proper sequence of cutting nails, and when to do so. Let’s look at the first word:

קמיצה (ring finger)
שמאל (left)
ימין (right)
אצבע (index finger)

In other words, when cutting your nails, start with your left ring finger, and then alternate every other finger. On your right hand, start with the index finger.5 Here is a diagram of the full sequence:

Even though there are some who are of the opinion that one need not be careful about cutting nails in this order, most write that ideally one should be careful about it.6

Cutting in Preparation for Shabbat

Now that you know the order of nail-cutting, you may wonder when to cut them. For this, we read the rest of the mnemonic:

באזהרה (warning)
לך (to you)
אתה (you)
תקוץ (cut)
יום (day)
רביעי (fourth)
והלאה (and on)
צפרניך (your nails)

In other words, cut your nails from Wednesday to Friday. Why? Because we trim our nails in honor of Shabbat, and the days from Wednesday to Friday have a connection7 to the upcoming Shabbat.8

Most, however, seem to be of the opinion that one should cut his nails on days that are even closer to Shabbat, i.e., Friday (or Thursday), when it is more discernable that the nails are being cut in preparation of the holy day.9

This is where things get a bit complicated. We are also warned that it is inauspicious to cut the nails of both the hands and feet on the same day.10 Therefore, one suggestion is to cut one’s toenails on Thursday and fingernails on Friday.11 Others, however, hold that one shouldn’t cut his nails on Thursday, since they start regrowing on the third day from when they were cut, and we don’t want them to start regrowing on Shabbat.12 After all, the whole point of cutting them to begin with is to honor the Shabbat.13

On a practical level, it is more important that the fingernails, rather than the toenails, be cut in honor of the Shabbat.14 Therefore, if both your fingernails and toenails are in need of being cut, you should cut your fingernails on Friday and your toenails on Thursday (or according to R. Chaim Noeh, Thursday night15).16 Additionally, if for whatever reason you know you won’t be able to cut your fingernails on Friday, you can cut them on Thursday.17

On the topic of cutting nails, here are some additional precautions (in brief):

Rosh Chodesh

Rabbi Yehudah he-Chassid (1150–1217) cautions that for mystical reasons, one should be careful not to cut his hair or nails on Rosh Chodesh (the Jewish New Moon).18

Chol Hamoed—Intermediate Days

The Ashkenazic custom is to not cut one’s nails on Chol Hamoed—the intermediate days of the Jewish holidays—because one should go into the holiday already well-groomed, and one should not push off grooming himself until he has some free time after the beginning of the holiday. 19

Burning Nails and Pregnant Women

While the above precautions about cutting one’s nails aren’t found in the Talmud, the Talmud does tell us that “the righteous bury their nails, the pious burn them, and the wicked carelessly discard them.” The explanation given for this is that nail clippings must be carefully disposed of, lest a pregnant woman step over them and miscarry20 (For more on that, see Nail Clippings and Pregnant Women.)

The Zohar explains that the forces of impurity are very much connected to the part of the fingernails that protrude above our fingers and are cut off.21 The Lubavitcher Rebbe explains that, on a personal level, the “fingernails” represent the part of us that can be used to prick and scratch someone. Thus, the importance of cutting our nails teaches us that before we interact with others, and specifically before rebuking someone, we need to “pare our own nails first.” We must ensure that any rebuke is given not to satisfy our own urge to criticize or belittle, but solely with the benefit of the person in mind.22