Genèse 17:26

In the same day, in the fourteenth year, was Abraham circumcised, and Ishmael his son.

IN THE SELFSAME DAY WAS ABRAHAM CIRCUMCISED. Rashi wrote: “In the selfsame day when Abraham completed his ninety-ninth year and Ishmael his thirteenth, they were circumcised.”
But what reason is there to mention this? Moreover, there is already general agreement with the words of Rabbi Eliezer, 27a. See also Seder Noach, Note 148, for fuller discussion of this point. who said that in the month of Tishri the world was created, and in Tishri the patriarchs were born, with the exception of Isaac who was born on Passover. Now Scripture says that the birth of Isaac will be at this time in the next year, and Rashi himself wrote further on in Seder Vayeira that the tidings were given on Passover. Consequently, Isaac was born on the next Passover!
Instead, the expression, In the selfsame day, means that on the very day he was charged with this commandment, he and the men of his house — three hundred and eighteen — and all those purchased with his money were circumcised. Scripture thus tells of Abraham’s distinction with respect to the fear of G-d, as well as that of all the men of his house, i.e., that they were all zealous, fulfilling their religious duty as promptly as possible.
Now the meaning of the phrase, was Abraham circumcised and Ishmael his son, is not that Abraham was circumcised first. Instead, it was Ishmael who was circumcised first, and then all the men of Abraham’s house, for it is so written: And Abraham took Ishmael his son, and all that were born in his house, and after that it says, And Abraham was ninety years and nine when he was circumcised. The reason for this was that Abraham was zealous concerning the commandment to circumcise them first, either doing it himself or perhaps inviting many circumcisers whom he supervised, and then he circumcised himself. Had he performed his circumcision first, he would have been sick or in danger on account of his advanced age, and he would then not have been in a position to concern himself with their circumcision.
Vayeira

בעצם היום הזה; we (the Torah) have already stated that Avraham circumcised all the members of his household on the very day G'd issued the command to him (verse 23). The Torah had not mentioned that he also circumcised himself and his son on that day. This is why this is added here, to tell us that he brooked no delay in performing the commandment both on his son and on himself. The meaning of the word נימול is that he performed the operation on himself and did not delegate anyone to do it as his agent. The use of a passive mode to describe that someone did something himself is not unique. We find it also in Samuel II 20,10 וכן עמשא לא נשמר, "and so Amassa did not protect himself."

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.]