An interesting question arises in the case of a Mohel who accepted Shabbat about an hour before sunset on Friday afternoon – as many people do during the summertime – and is then asked to perform a Berit for a baby born the previous Friday. This baby requires circumcision that day, Friday, as it is his eighth day, but as this Mohel had already accepted Shabbat, it is unclear whether he is allowed to perform the Berit. As we know, a Berit Mila is performed on Shabbat for a child who was born the previous Shabbat, because circumcision on the eighth day overrides the prohibitions of Shabbat. It is questionable, however, whether this law would also permit the Mohel in the case described above to perform the circumcision on Friday, after he had accepted Shabbat, for a child born on Friday.
This issue touches upon the conceptual question of how to understand the special law permitting a Berit on Shabbat. On the one hand, we might say, very simply, that the Misva of Mila Bi’zmanah – circumcising a child on his eighth day – overrides the prohibitions of Shabbat. Meaning, the Torah viewed the performance of a Berit Mila on a child’s eighth day as important enough to override the prohibitions of Shabbat. According to this perspective, it would seem that the Mohel in our case should perform the Berit, as the need to circumcise this child that day overrides the Shabbat laws. On the other hand, one might argue that the Torah mandated performing a Berit when the eighth day falls on Shabbat. It is not the Misva of Mila Bi’zmanah per se which overrides the Shabbat laws, but rather the specific instance of an eighth day which falls on Shabbat. According to this understanding, Shabbat violation is permitted only for a circumcision performed on Shabbat, not for a circumcision performed on Friday.
Different positions have been taken surrounding this question, and there does not appear to be a clear consensus (soba semahot page 174).
If the infant’s eighth day was a weekday, and, for whatever reason, the Berit was not performed until after sundown, the Berit may still be performed, until 13.5 minutes after sunset. Moreover, the Berachot may be recited in such a case. One reason for this ruling is that this is a situation of "Sefek Sefeka" (a "double doubt"): it is uncertain whether the 13.-minute period is considered daytime or nighttime, and, secondly, Halacha might perhaps follow the view of Rabbenu Tam (France, 1100-1171), that the uncertain period of "Ben Ha’shemashot" actually begins later than sunset. Moreover, a circumcision performed at night is valid, despite the fact that it should optimally be performed during the daytime. In light of these factors, a baby that was not circumcised before sundown on his eighth day may be circumcised during the 13.5-minute period immediately after sundown.
It should be noted, however, that this does not apply on Friday, when it would certainly be forbidden to perform a Berit after sundown. Since we begin observing Shabbat at sundown, it becomes forbidden to perform a Berit at this point. The possibility that the day has not yet ended does not allow risking a potential act of Shabbat desecration.
Summary: If a Mohel accepted Shabbat before sundown, and is then asked to perform a Berit on an infant who requires a Berit Mila on Friday, it is questionable whether he is allowed to perform the Berit. If a child was not circumcised before sundown on his eighth day, which is a weekday, he may still be circumcised during the 13.5-minute period after sundown, but not if this is Friday.
