Cain left the presence of Hashem and settled in
the land of wandering, east of Eden.
(Bereishis 4:16).
After Cain left Hashem's judgment for killing Hevel, Adam met him
and asked 'How did your case go?' Cain replied, "I repented
and Hashem forgave me." Thereupon Adam began to cry and beat
his face, "I did not know how great the power of teshuvah
is." Adam then arose and exclaimed: "A Psalm, a song
for the Shabbos Day. It is good to confess to Hashem."
(Midrash Rabbah 22:13)
The obvious question is why did Adam mention a Posuk that deals
primarily with Shabbos; what does Shabbos have to do with
teshuvah? The answer is that because on Shabbos we are free from
all worldly pursuits, it is the best time to do teshuvah. It is
the single day of the week during which we are free to
contemplate our goals and actions in this world.
With this idea, Rav Nosson Levine, the father of the Reisha Rav,
explained the following Gemarah:
One who sleeps seven days without a dream is called evil.
(Berachos 55)
Two points require clarification. First, why did the Gemarah
choose seven days rather than some other number of days? Second,
why should a lack of dreams indicate that a person should be
called evil? In response to the first question, he explains that
every seven-day period includes a Shabbos. Thus, when the Gemarah
mentions one who has slept seven days without a dream, it means
that he has passed through Shabbos without a dream. In response
to the second question, Rav Nosson Levine explains that the Dream
does not refer to an actual dream but to the thought that life is
short like a dream. With those two points, we can understand the
Gemarah as follows: If a person passes through Shabbos without
contemplating that his life is short like a dream, he is called
evil. Because Shabbos is the day during which one is free from
all worldly pursuits, it is a day that is ideal for Teshuvah. If
one lets that opportunity slip by, the Gemarah is telling us that
he is called evil.