I simply wish to quote the last paragraph of Tom Hale’s commentary on Joel. I believe He provides an excellent summation and application:

‘As we think back on this small but very important book, let us keep Joel’s message in mind: a day of judgment – a ”day of the Lord” – is coming, and we do not know when. There may be a number of such ”days” coming – judgments upon our present nations and communities, and upon ourselves. Joel’s message to us is that we should repent and cry out to God for mercy before it is too late. God is patient; He will wait a long time, but He will not wait forever. The ”invasion of locusts” can come at any time. We need to be ready for the day of the Lord; it will come like a thief (2 Peter 3:10). In view of this, Peter asks:…what kind of people ought (we) to be? (2 Peter 3:11).’ (The Applied Old Testament Commentary’, p.1239.