Reference for Shabbat 263:1
שכן אם עבר זמנה בטלה אלא היינו טעמא דרבי אליעזר דאמר קרא (ויקרא יב, ג) וביום השמיני ימול בשר ערלתו ואפילו בשבת
[they may supersede the Sabbath] because if their time passes they are annulled!<span class="x" onmousemove="('comment',' They must be performed at a certain time or not at all. But circumcision, though obligatory for the eighth day from birth, can and must be performed afterwards if not done then. ');"><sup>1</sup></span>
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