Rodzaju 17:21 Komentarz: Radak, The Midrash, Daat Zkenim & Chizkuni

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.”