Jamieson, Fausset and Brown on Joel 2:30-31

30-31. As Messiah's manifestation is full of joy to believers, so it has an aspect of wrath to unbelievers, which is represented here. Thus when the Jews received Him not in His coming of grace, He came in judgment on Jerusalem. Physical prodigies, massacres, and conflagrations preceded its destruction [Josephus, Wars of the Jews]. To these the language here may allude; but the figures chiefly symbolize political revolutions and changes in the ruling powers of the world, prognosticated by previous disasters (Am 8:9; Mt 24:29; Lu 21:25-27), and convulsions such as preceded the overthrow of the Jewish polity. Such shall probably occur in a more appalling degree before the final destruction of the ungodly world ("the great and terrible day of Jehovah," compare Mal 4:5), of which Jerusalem's overthrow is the type and earnest.