Why did Hashem repeat, "v'Es Brisi Akim Es Yitzchak"?
Rashi #1: To teach us that Yitzchak was sanctified from birth. 1
Rashi #2: To teach us a Kal va'Chomer from the "son of the maidservant" (Yishma'el), whom Hahsem blessed in 17:20; how much more so the "son of the mistress" (Yitzchak)! 2
Hashem promised that Yitzchak would be born, "... at this time, in the following year (la'Mo'ed ha'Zeh, ba'Shanah ha'Acheres)." But two days later, (on the third day from the Milah,) the Mal'achim who visited Avraham again said, "I shall return to you at this same time (ka'Eis Chayah) ..." (18:10). [Likewise, Hashem said on that occasion, "At the appointed time (la'Mo'ed) ... " (18:14).] How can both days be correct?
Rashi (on 18:10), according to Gur Aryeh (loc.cit.): Yitzchak would be born precisely one year after the Milah, as implied by this Pasuk. The verse "ka'Eis Chayah" (18:10) stated two days later, refers to the Pesach season, and not the exact calendar date. 1 [Likewise, "la'Mo'ed" (18:14) refers to the date that had already been designated in our verse, the date of the Bris Milah (Rashi to 18:14).]
Refer to 17:21:151.1.
One opinion in the Gemara (Rosh Hashanah 11a) is that Hashem promised on Sukos that "la'Mo'ed" - "at the next Festival" (18:14), Sarah would have a son; and Yitzchak was born the following Pesach. According to that opinion, how can we explain our Pasuk, which states that Yitzchak would be born "at this time, in the following year"?
Maharal (Chidushei Agados Vol. 1, p. 98, to Rosh Hashanah 11a): In our verse, Hashem is not designating the date on which Sarah would give birth, but promising that by this date next year, Sarah will already have given birth. According to that opinion in the Gemara, the Milah was in Tishrei, and Yitzchak was born approximately six months later, in Nisan. 1
Riva (to Bereishis 18: 10,12): According to Rashi, the Milah and the subsequent visit from the angels took place on Pesach, and they informed Avraham and Sarah that Yitzchak would be born the next year. But come Tishrei, Sarah had still not conceived, and it was at that point she expressed disbelief that there would be time to become pregnant and give birth by Nisan. Hashem sent her a rebuke, and reiterated His promise that Yitzchak would be born on the next festival, Pesach. 2
Maharal adds that Avraham's Bris Milah was on Yom Kippur (Tosfos loc. cit., citing Pirkei d'Rebbi Eliezer; refer to 17:26:1:3). He was informed that very day that Yitzchak would be born that year, for it is on Yom Kippur that the judgment for the coming year is sealed.
Riva is trying to resolve our verse, "ba'Shanah ha'Acheres," which he understands as, "in the next calendar year." This answer is necessary according to Rebbi Eliezer. According to R. Yehoshua, who says that we count years from Nisan, Pesach is already considered, "the next year," and we can accept Answer #1
One opinion in the Gemara (Rosh Hashanah 11a) is that the Mal'achim visited Avraham on Sukos; and Hashem promised that a son would be born "la'Mo'ed" (18:14), meaning, "on the [next] Festival," i.e. Pesach. Why doesn't that opinion explain our verse in the same way?
Maharal (Chidushei Agados Vol. 1, p. 98, to Rosh Hashanah 11a): The later verse (18:14) does not specify which Festival is intended, so it must mean the next festival. Our verse, however, says "la'Moed ha'Zeh," on this festival. 1