If a child’s eighth day falls on Shabbat, the Berit Mila is performed on that day, as the Misva of Berit Mila on the eighth day overrides the Shabbat prohibitions. However, as the Shulhan Aruch rules (Orah Haim 331, Yoreh De’a 266), an exception is made in the case of an inexperienced Mohel. If a Mohel had never before performed a Berit, then he is not permitted to perform a Berit on Shabbat. The reason is that because of the Mohel’s inexperience, it is possible that he might not perform the procedure properly, in which case he will have violated Shabbat, as the Shabbat prohibitions are waived only for the purpose of performing a proper Berit Mila. Therefore, a Mohel who has never before performed even one Berit may not perform a Berit on Shabbat.
This applies also to the baby’s father. If the father wishes to perform his son’s Berit, which is taking place on Shabbat, but the father had never before performed a Berit, he may not serve as the Mohel.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) adds that this applies even to a Mohel who has previously performed the first stage of circumcision – the cutting of the foreskin – but not the second stage, that of Peri’a (removing the thin layer of skin underneath the foreskin). If he has never before performed Peri’a, then he may not do so for the first time on Shabbat. However, one who has never before performed Mesisa (drawing blood from the wound after the circumcision) may do so for the first time on Shabbat. Since the Mesisa is a relatively simple procedure, there is no reason for concern that it might not be done correctly.
A Mohel who claims that he has experience performing Berit Mila may be trusted and thus allowed to perform a Berit on Shabbat, and he is not required to provide any sort of proof or present witnesses.
Summary: Somebody who has never before performed a Berit may not do so for the first time on Shabbat. Although a Berit Mila on a child’s eighth day may be performed on Shabbat, the Mohel in such a case must have performed at least one Berit previously.