The Performance of Brit Milah on Shabbat
The obligation of Brit Milah appears at the beginning of Parshat Tazria.
While we first encounter this ritual in Parshat Lekh L'cha, Am Yisrael are now obligated to perform Brit Milah on the eighth day.
וּבַיּוֹם, הַשְּׁמִינִי, יִמּוֹל, בְּשַׂר עָרְלָתוֹ.
And in the eighth day the flesh of his foreskin shall be circumcised.
What emerges from this short pasuk is an interesting discussion about the power given to the Chachamim to the authority of Chazal to allow what may appear as a violation on shabbat or forbade a directive from the Torah if it could involve a violation of Shabbat.
Chazal derive from the pasuk in our parshat that an eight day old baby boy is given a brit milah even on the Shabbat. In masekhet Shabbat, 132a we learn that even though causing a bloody wound is a violation on Shabbat, this obligation of performing Brit Milah on the eighth day requires us to suspend the laws of Shabbat.
Many commentators to the Gemara question why Chazal did not prohibit one's performance of Brit Milah on Shabbat, like other mitzvot are prohibited to do on Shabbat. For example, we do not take a lulav/etrog bundle on Shabbat, we do not read Megilat Esther when Purim is on Shabbat or blow the Shofar on Rosh Hashana when it falls out on Shabbat. For lulav, Megilah and shofar, there was a concern of carrying these items through a r'shut haRabim, through a public domain on Shabbat in order to perform the mitzvah or improve in one's ability to perform the mitzvah by visiting an expert to assist in preparing to perform the mitzvah.
However, Chazal not rule that there is a concern of carrying the knife for Milah through a r'shut haRabim - through a public domain. Do we not have a similar concern with regard to a Mohel carrying his knife to perform milah?
The Taz, the Turei Zahav (Rabbi David haLevi Segal 16-17th century), the commentary on the Shulchan Aruch, speaks about the principle that Chazal has been given the authority by the Torah to make "fences" around the Torah - even if it means suspending a mitzvah to protect the observance of a mitzvah. What we would call a "g'zaira".
However, if the Torah explicitly states a performance of a mitzvah formulated in the Torah, Chazal's power is limited and cannot override that law. The Taz explains that is why we are permitted to blow the Shofar on Rosh Hashana during the weekday, because the Torah explicitly stated to blow the shofar on the day of Rosh Hashana. Therefore, Chazal may not do away with Shofar blowing on the weekday.
The Taz compares this to circumcision. The Torah explicitly teaches that there should be circumcision on the eighth day of a baby's life. If the eighth day is a Shabbat,not performing the mitzvah (obviously with a baby who is healthy enough for the circumcision), would end up being a direct violation of a Torah commandment. Chazal were not empowered to overturn that mandate over their concern for Shabbat observance.
Wishing you all a Shabbat Shalom.