Kitzur Sh’lk, fol. 51, col. 1.

A man should study less on Friday, that he may occupy himself with the preparation for the Sabbath.  And accordingly we find in the Gemara that some of the great and esteemed sages occupied themselves on that day in preparing what was needed for the Sabbath.  Therefore, though one may have many servants to wait upon him, it is a great merit personally to prepare for the wants of the Sabbath in order thus to honor it; and let him not think it derogatory to his own honor to honor the Sabbath thus, for it is his honor to honor the Sabbath.  It is written of H’A’ree of blessed memory, that he was in the habit of sweeping away the cobwebs in his house (in honor of the Sabbath), and it is well known to the initiated what a wonderful mystery it is to abolish the unclean spirits from the house, “And this is enough for him that understands.”

Ibid., fol. 61, col. 1.

One should trim his finger-nails every Friday, never on Thursday, otherwise the nails will commence growing on the following Sabbath.  He should pare the nails of the left hand first, beginning at the fourth finger and ending with the thumb; and then he should pare the nails of the right hand, beginning with the thumb and ending with the fourth finger; he should not vary the following order:  4th, 2d, 5th, 3d, 1st of the left hand; then the 1st, 3d, 5th, 2d, 4th of the right hand.  Never pare two (contiguous) fingers one after the other, for it is dangerous, and it also impairs the memory.  The reason and mystery about the order for paring the nails are well known to the expert.

Kitzur Sh’lh.

In the Zohar it is explained that the benefit of immersion on Friday amounts to the restoration of the soul to her proper place, for he who is bodily unclean has no soul.

Ibid., fol. 61, col. 2.

Before entering the plunging-bath, he is to repeat (Gen. i. 10), “And God called the dry land earth, and the gathering together of the waters called He seas.”  When he stands in the water he is to repeat seven times (Ps. li. 10), “Create in me a clean heart, O God, and renew a right spirit within me,” for the initials of “Create in me a clean heart,” form the word “to dip,” i.e., to immerse.  For it is through immersion that the unclean spirits and the “other side,” are separated from him, and he becomes a new creature by examining and confessing his (evil) deeds, and forsaking them, and by engaging himself in repentance, and immersing himself, and meditating on elevating subjects, and especially so if he has immersed himself fourteen times.

Ibid., fol. 61, col. 1.