הֲלֹא־חֹ֛שֶׁךְ י֥וֹם יְהוָ֖ה וְלֹא־א֑וֹר וְאָפֵ֖ל וְלֹא־נֹ֥גַֽהּ לֽוֹ׃
Shall not the day of the LORD be darkness, and not light? Even very dark, and no brightness in it?
Read halakhic rulings (Jewish law) on Amos 5:20: and classical halakhic sources.