The nineteenth perek (=chapter) of Massekhet Shabbat, which begins on today's daf (=page) focuses in its entirety on the laws of berit milah – circumcision – generally and on its performance on Shabbat specifically.
The Sages received an oral tradition that the mitzvah of circumcision overrides Shabbat when it is performed at its proper time, on the eighth day after birth. Although all the Sages accepted this tradition, they disputed many details of its application. Such details include the handling of borderline cases, the precise definitions of the act of circumcision, and the actions necessary for its performance.
Several issues arise with regard to circumcision on Shabbat. The first question pertains to situations in which it is not entirely clear whether there is a mitzvah to circumcise the child or whether the proper time for circumcision has arrived. Some Sages are of the opinion that circumcision overrides Shabbat only if there is absolutely no doubt as to its obligation. Others hold that in some circumstances an uncertain obligation to circumcise does override Shabbat, and these Sages debate which conditions are required.
Another issue is whether the preparations for circumcision override Shabbat. The medical attention the baby will require immediately after the circumcision does override Shabbat, as his life could be endangered if he does not receive it. The Sages consider whether actions to prepare for the circumcision, such as bringing the knife and preparing it for use, also override Shabbat, and if so, which preparations are considered essential in this regard.
Another essential issue pertains to how broadly circumcision overrides Shabbat. Does circumcision override Shabbat at every stage of the operation no matter how it is performed, and is anyone involved in circumcision exempt from Shabbat prohibitions? Or perhaps circumcision overrides Shabbat only when one completes the mitzvah properly and in its entirety? To address these questions it becomes necessary to clarify the scope of the mitzvah and to identify its obligatory components.