Adam Clarke on Joel 2:31

The sun shall be turned into darkness - The Jewish polity, civil and ecclesiastical, shall be entirely destroyed.

Before the great and the terrible day of the Lord come - In the taking and sacking of Jerusalem, and burning of the temple, by the Romans, under Titus, the son of Vespasian. This was, perhaps, the greatest and most terrible day of God's vengeance ever shown to the world, or that ever will be shown, till the great day of the general judgment. For a full view of this subject, I wish to refer the reader to the notes on Matthew 24.