Why did Yitzchak mention that he did not know when he would die?
Rashi: Because when a person reaches within five years of the age when his parents died, he needs to take into account the likelihood of death occurring. His mother Sarah died at the age of 127, and he was now 123. 1
Seforno: The point he was making was that the closer to one's death that one gives a Berachah the more effective the Berachah will be. 2
Rashbam: He was explaining to Esav that his current request was based on the fact that he wanted to beat the Mal'ach ha'Maves, as it were, and to bless him with the Berachos over which he had a mandate - the Berachah of Avraham and the Berachah of Eretz Yisrael, 3 while he was still alive.
Alshich, Ha'amek Davar: Yitzchak thought that his sudden blindness is a sign that he is about to die.
Yitzchak should have been concerned a year ago! Perhaps indeed, his concern began last Pesach, when he turned 122; he decided to wait until Erev Pesach, just before he turns 123, the best day for the Berachah (refer to 27:1:2:1). Rashi said that he was 123 based on when Esav married Machalas, which was surely at least one day later. Alternatively, initially Yitzchak assumed that his two years in Gan Eden (refer to 25:27:1.1:2) do not count, so it is as if he is only 121. Later, he was unsure, and decided that to be safe, he will bless him now. (PF)
This is like we find by Yaakov Avinu and Moshe Rabeinu.
I do not understand this, for Yitzchak did not give these Berachos when he thought that he is blessing Esav! (PF)
QUESTIONS ON RASHI
Rashi writes: "'I do not know the day of my passing' - When a person reaches the age at which [either of] his parents passed away, he should be fearful - for five years beforehand, and five years afterwards." Why is this?
Gur Aryeh: Every ten years, a person changes; see Mishnah Avos (5:21). Ten years comprise one unit of time.