Mitzvot sometimes conflict with each other. The rule is Aseh docheh lo ta’aseh – “a positive law overrides a negative law”. Hence though we might think that a circumcision would not take place on Shabbat, the opposite is the case; the positive law of b’rit milah overrides the negative law of not working on Shabbat.
The Chafetz Chayyim told a story connected with this week’s reading which bears on this rule. Quoting the verse, Umal’tem et or’lat l’vav’chem, “You shall circumcise the foreskin of your heart” (Deut. 10:16), he said that a rabbinic sage opened the door of the Bet Midrash one Shabbat morning to find the congregation sitting and learning Torah before the service.
He said, “One would expect that on Shabbat morning everyone would think first of praying and then going home to eat, but the congregation have restrained themselves from their Shabbat observance in order not to lose a moment from their Torah study. Truly, I see milah (the mark of piety and learning) overriding Shabbat!”