וביום השביעי שבת שבתון ... כל מלאכה לא תעשו...
The מדרש says that אדם הראשון met קין and asked him what happened with his דין. Kayin told him that he did תשובה. Adam couldn't believe that this was the כוח התשובה and he didn't even know about it! Adam got up right then and said "מזמור שיר ליום השבת"
What is this מדרש coming to tell us?
Hagaon Horav Tzvi Hersh Rabinovitch (אב"ד קובנא) tells of a beautiful משל.
He tells of a very large store that closes one day a week in order to make all the necessary calculations and to keep track of the income and expenses. However, if the store goes bankrupt from all the debt and losses, they no longer need the day off to make any calculations since they will be claiming bankruptcy.
The understanding of אדם הראשון at first was that if someone does an עבירה, there is no תקנה for him and no way to correct it. He is in a spiritual bankruptcy. Therefore there is no reason for him to set a day to make calculations. However once אדם הראשון heard about the כוח התשובה, he said מזמור שיר ליום השבת. Shabbos is a very good day to make a חשבון הנפש and to pay up חובות from the week through living a life being connected to the רבונו של עולם.
We can all use the unique opportunity and beautiful gift that the רבונו של עולם gave us to spend the time on שבת קודש being connected to Him.
Rabbi Mayer Shimon Kalmanowitz