The Midrash relates the following incident: One day while traveling, Adam Harishon came across his son Kayin on the road. Adam was shocked at discovering that his son Kayin was still alive for only a short while before, Kayin had been banished from the rest of mankind for his grievous sin of murdering his brother, Hevel. Adam asked Kayin, "why are you still alive? Didn't your sin warrant a death sentence"? Kayin replied, "I did Teshuva" (I repented). Adam then proceeded to pronounce the psalm "Mizmor Shir L'yom Hashabbos" (A psalm for the Sabbath day).

Adam's response is seemingly very strange. What was it about this psalm that Adam felt it appropriate to recite it in this context? What did their dialogue have anything to do with Shabbos?

Back in Europe, when Jews lived in shtetls, most were poor and led very difficult lives. During the course of the week, these Jews could be often be seen walking the streets in tattered clothing, krechtzing and kvetching (moaning and complaining) about how hard their lives were. And yet the moment Shabbos arrived a metamorphosis took place. Everyone suddenly became princely and honorable, radiating a happiness that was non-existent during the rest of the week. Though their clothes had not changed and neither had their financial situations, there was a tangible change in their demeanor. They were no longer the lowly souls they were before. Such is the power of the Shabbos.

When Kayin told Adam Harishon that he had done teshuva, what he was actually saying was that he was no longer "Kayin the murderer". His teshuvah had actually qualitatively changed him to the point where he was no longer the same person he had been before. He was an entirely different Kayin and thus no longer warranted the punishment of death.

When Adam recognized that such was the power of Teshuva, he immediately associated it with the power of Shabbos. Both of these mitzvos seem to be capable of effecting a change in the essence of a person. Both can take the lowliest souls up to the loftiest heights. And thus it was appropriate for Adam to respond to Kayin with the psalm, Mizmor Shir L'yom Hashabbos.