570. What May Be Done on Shabbos

Milah 2:5

If a child’s skin hangs very loosely, or if he is so fat that it appears as if he is not circumcised, then he should be observed when erect. If he is visibly circumcised at that time, nothing else need be done, but the flesh on the sides must be corrected so that he no longer appears to be uncircumcised. If he does not appear to be circumcised even when he is erect, the loose flesh on the sides must be removed so that the tip is revealed when he is erect. This law was instituted by the Sages; under Torah law, if one was circumcised, he need not be circumcised again, even if he looks uncircumcised.

Milah 2:6

Anything required for the bris may be done on Shabbos, including: the steps of removing the foreskin, peeling the membrane, and suction; removing strands of skin that invalidate the bris even if the mohel has stopped performing the bris; going back to remove strands of skin that do not invalidate the bris if the mohel has not stopped performing the bris; bandaging the wound. Preparing the items needed for the bris does not override the Shabbos prohibitions against acts of labor. Therefore, if one is unable to find a knife, a knife may not be made on Shabbos, nor may it be carried from another place; one may not even transport it from one courtyard to another in the same alley without an eiruv. Even though the requirement to have an eiruv is of rabbinic origin, it is not overridden in order to bring a knife because this could have been done before Shabbos.