Joel 2:31 - Matthew Poole's English Annotations on the Holy Bible

Having mentioned the prodigies which were to be wrought on earth, now the prophet specifieth what shall be done in heaven, where the great luminaries shall be wonderfully affected. The sun shall be turned into darkness; shall be greatly obscured; shall seem to be turned from a body of light to mere darkness. The moon into blood; either by eclipse, or by the intervention of vapours drawn up from the places where was great slaughter and effusion of blood; however as to manner, it is most certain as to the event; the moon shall seem to be blood. Before the great and the terrible day of the Lord come; great to all, terrible to the rejecters and persecutors of Christ. This day was the day of Jerusalem's destruction, and burning of the temple, and slaughter of the Jews, for their violence against and murder of the Messiah, for their sins against the gospel: this was fulfilled partly in the devastation of Jerusalem, but shall fully and finally be fulfilled in the day of judgment, and at the consummation of the world.