Cain left the presence of Hashem and settled in
the land of wandering, east of Eden.
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.
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.