ספר שיחות מוהר"ן אות - רפז

I heard that once he said to a certain person in these words (Avos 2:10; Shabbos 153a): "Repent one day before your death" — and he drew out and prolonged the words "one day." And his meaning was about what brings a person down — what has already passed and what is yet to come. That is: when he sees that in previous days he did not serve Hashem as he should — and also in days yet to come there is no certainty that he will serve Him properly — therefore he despairs, Heaven forbid, and falls completely. And the remedy for this is: "Repent one day before your death" — and he drew out the words "one day." The meaning: today's single day is sufficient. One should not concern oneself with what has passed and what is yet to come — only with the present day. "Repent one day" — this one day is enough and sufficient to return to Hashem Yisburach in truth. As it is written (Psalms 139:16): "All days were fashioned — and for it, one of them." For each and every day is complete in itself, and through this one day alone one can merit much.