Gen 17:26

“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”

“In the selfsame day was Abraham circumcised, and Ishmael his son. This is repeated, that it might be taken notice of that both were circumcised according to the command of God, and on the very day in which it was given. Jarchi observes, it was in the day, and not in the night; which shows, says he, he was not afraid of the Heathen, and of mockers; and that his enemies, and the men of that generation, might not say, if we had seen him, we would not have suffered him to be circumcised, and keep the commandment of God: and some of the Jewish writers (e) fable, that he was circumcised on the day afterwards appointed by Moses for the day of atonement, and that in the place where he was circumcised the altar was built; but all this is without any foundation. This affair was transacted, according to Bishop Usher (f), A. M. 2107, and before Christ 1897. (e) Pirke Eliezer, ut supra. (c. 29.) (f) Annales Ver. Test. p. 8.”