וְאֶת־בְּרִיתִ֖י אָקִ֣ים אֶת־יִצְחָ֑ק אֲשֶׁר֩ תֵּלֵ֨ד לְךָ֤ שָׂרָה֙ לַמּוֹעֵ֣ד הַזֶּ֔ה בַּשָּׁנָ֖ה הָאַחֶֽרֶת׃
Mas yo estableceré mi pacto con Isaac, al cual te parirá Sara por este tiempo el año siguiente.
Radak on Genesis
למועד הזה "at this time." (anniversary). We do not know precisely on what date G'd had granted Avraham this prophetic vision.
The Midrash of Philo
Daat Zkenim on Genesis
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.”