Kommentar zu Bereschit 27:2

לא ידעתי יום מותי I KNOW NOT THE DAY OF MY DEATH — R. Joshua the son of Korcha said: When a person approaches the age at which his parents died he may well be anxious five years before and five years after. Isaac was then one hundred and twenty three years old and he said, “Perhaps I shall only reach the age of my mother who died at the age of 127 and I am now within five years of her age: therefore, I know not the day of my death —I may only reach the age of my mother or it may be the age of my father” (Genesis Rabbah 25:12).

הנה נא, I beg of you, due to my old age and the fact that I want to bestow on you the blessing which is within my power to bestow, the blessing of Avraham concerning inheriting this land.

לא ידעתי יום מותי. Although the blessing will prove more effective the closer the one dispensing is to death, as was the case in the blessings dispensed by Yaakov and Moses, seeing that their souls were already closer to heaven at the time when they dispensed the blessing, Yitzchok was not aware of when he would die so that he could to time his blessing accordingly.