פּירוש אויף בראשית 17:23

בעצם היום IN THE SELFSAME DAY — on the very day when he received the command of circumcision: by day and not by night — he was afraid neither of the heathens nor of the cynics. And he did it in broad daylight in order that his enemies and other people should not say, “If we had seen him we would never have allowed him to practise circumcision and carry out the commandment of the Omnipresent (Genesis Rabbah 47:9).

בעצם היום הזה, “on this very day;” According to Rabbi Gamliel, Avraham sent a messenger to Shem, son of Noach, and invited him to circumcise him and his son Ishmael. Not only that, but the date was the tenth of Tishrey, i.e. Yom Kippur which during that year occurred on a Sabbath. He derived this from the use of the expression בעצם היום in Leviticus 23,29. [Presumably Avraham knew that performance of this commandment on the eighth day of a newborn child overrides the prohibitions of the Sabbath and even Yom Kippur. Ed.] This is why every year on the recurrence of that date, G–d reminds Himself of Avraham’s having performed this commandment and it is a factor contributing to atoning for all of our sins on that Day. In B’reshit Rabbah, there is a version describing that after Avraham had already had the knife and his foreskin in his hand to perform the circumcision on himself, he became afraid that he might tremble due to his advanced age. G–d thereupon assisted him and guided his hand; this is why we say in our daily prayers in the morning when relating to this event: וכרות עמו הברית, “He cut the covenant with him (Avraham).” The quotation is based on Nechemyah 9,8. The usual translation of the word, inaccurate is: ”He concluded” Ed.]

בעצם היום הזה, without any delay.

וימל AND HE CURCUMCISED — This is a Kal form (active).