Why does the Mishnah advise us to “repent one day before your death”?  How is one to know the day of his death?

Rabbi Eliezer taught, “Repent one day before your death.”

His disciples asked him, “But does a man know on what day he will die?”

“That is exactly the point!” he replied. “Let a man repent today lest he die tomorrow, and in this way he will live all his days in repentance.”

An alternative reason for doing so is to be found in Sefer Maharil: “One should repent every day..., because in the days of Mashiach repentance will not be acceptable. (Similarly, converts will not be accepted then, for a person seeking conversion at that time will be prompted not by the love of G-d but by a desire to share the joyful lot of the Jewish people.) And since we are expecting to be saved every day, whoever does not repent before Mashiach comes (Speedily in our own days!) will no longer be able to repent; he will remain (G-d forbid) with his sins.” If so, then instead of repenting out of a dread of the day of death, surely it is preferable to repent out of a hopeful anticipation of the coming of Mashiach!

(However, this should not be taken to mean that repentance will be of no avail when Moshiach comes.  Rather, the reward of doing Teshuva in the time of exile, when there is a concealment of G-dliness, is far greater than doing Teshuva after Moshiach comes, when G-dliness will be revealed and Teshuva will be a natural response, requiring far less effort on our part.  As our sages say, “according to the effort is the reward.” )

Avot 2:10.  Shabbat 153a.  Maharil p. 42.  Yevamot 24b.  Based on an address of the Rebbe, Shabbat Parshat Vayeilech, 1958.  Exerpted from From Exile to Remption, published by Sichot in English.