הַשֶּׁ֙מֶשׁ֙ יֵהָפֵ֣ךְ לְחֹ֔שֶׁךְ וְהַיָּרֵ֖חַ לְדָ֑ם לִפְנֵ֗י בּ֚וֹא י֣וֹם יְהוָ֔ה הַגָּד֖וֹל וְהַנּוֹרָֽא׃
The sun shall be turned into darkness, And the moon into blood, Before the great and terrible day of the LORD come.
Read kabbalistic (mystical) interpretation of Joel 3:4: and classical mystical sources.