פירוש על בראשית 17:21

למועד הזה "at this time." (anniversary). We do not know precisely on what date G'd had granted Avraham this prophetic vision.

ואת בריתי אקים את יצחק, “and My covenant I will establish with Yitzchok;” the word אקים is to be understood as an acrostic from the phrase: אשר קידש ידיד מבטן, a phrase recited after every circumcision, meaning: “who is well beloved already when he emerges from the womb.”

למועד הזו לשנה האחרת "at this season next year;" Rav Huna said in the name of Rav Idi: that year was a leap year [in which an additional month of Adar was added before Nisan], meaning "מועד" and "מועד" was one year apart from each other, and he calls it "next year" because of the additional month it has in it.

בשנה האחרת. G'd informs Avraham that a year will elapse between the time he received this message and its realization. It is as if the Torah had written that Sarah would give birth at this time in the following year. He did not tell him when she would become pregnant, nor the length of time of her pregnancy. Our sages in Rosh Hashanah 10 assume that the birth occurred on Passover (the date on which Passover would occur in the future) seeing that Sarah became pregnant on Rosh Hashanah.