Musar do Jeremiasza 2:5

Midrash Kohelet, quoting Rabbi Yehudah bar Simon, is of the opinion that the seven days of impurity a woman experiences in connection with menstruation correspond to the seven times Solomon referred to הבלים, futilities, in Kohelet 1,2. [When the word הבל appears in the singular it refers to a single "futility," whereas when it appears in the plural it refers to two "futilities." Ed.] This means that in view of the sin committed by Adam and Eve everything which was created during the seven days of Creation is futile. Creation experienced another damaging blow when Cain murdered his brother הבל. Man indulged in the pursuit of futile objectives, וילכו אחרי ההבל ויהבלו, "They went after delusion and were deluded" (Jeremiah 2,5).