Brit Milah ברית מילה
Circumcision on the eighth day — the sign of the covenant made with Avraham and his descendants forever.
Brit milah was given to Avraham in 2047, at ninety-nine: «it shall be a sign of the covenant between Me and you». He circumcises himself, the thirteen-year-old Yishmael and his whole household in one day. Yitzchak is the first circumcised on the eighth day, and that has been the rule since — even on Shabbat or Yom Kippur; only the child's health delays it.
The sages note the paradox: the sign of the covenant is made by a human being, on the body — the completion of creation handed to man. Midrash records Turnus Rufus asking Rabbi Akiva whose works are finer, God's or man's, and Rabbi Akiva answering with ears of grain and a loaf of bread.
Antiochus and Hadrian both forbade it, and it was precisely what people kept doing in secret.