전도서 11:8 해설: Rashi, Kohelet Rabbah & Tze'enah Ure'enah

Let him rejoice in all of them. Let him be happy with his lot, provided that he remembers the days of darkness and improve his deeds so that he be saved from them; and these are the days of eternal death, they are the days of the wicked.

“For if a man lives many years, let him rejoice in all of them, and remember the days of darkness, as they will be many. Everything that is coming is vanity” (Ecclesiastes 11:8).
“For if [a man lives] many years,” “let him rejoice” in the joy of the Torah; “and remember the days of darkness” – these are the bad days, “as they will be many.” The Torah that a person studies in this world is vanity relative to the Torah of the Messiah.

For they will be many. In those days, more than the days of life.²³I.e., the days of darkness, when he will be in his grave, will be many more than the days of his life. (Metsudas Dovid)

All that comes. Upon him will be punishment and darkness. “הֶבֶל” sometimes is an expression of punishment and troubles, as in, “for he [the stillborn child] comes with troubles הֶבֶל and departs in darkness.”²⁴Above 6:4.