Joel was a prophet who probably lived sometime between 850-740 B.C., however, it is not certain. We know very little about the prophet Joel. It is not likely that the Joel who wrote this book is the same Joel mentioned somewhere else in the Bible. Because Joel did not state the nations of Assyria or Babylon, it is hard to tell when he wrote. He seemed to be familiar with the country of Judah, so we can just assume that he preached there.
Joel's message centers around what he calls the 'day of the Lord'. We know that the 'day of the Lord' refers to a day or time of judgment when the Lord delivers rewards and punishments. And while we usually refer to the Second Coming as the day of the Lord, there are other days of the Lord. For example, when Israel was conquered by the Assyrians that was the day of the Lord's judgment on Israel. On an individual level, the day of the Lord may be the day a person dies. However, if we refer to the 'day of the Lord' in our latter day application, we usually regard to it as the Second Coming of Jesus Christ. In the Book of Mormon, Moroni told Joseph Smith that a passage in Joel 2:28-32 would 'soon' be fulfilled.1
As we read and study the book of Joel there are three important principles to read and ponder: 1) Terrible wars, desolations, and natural disasters will destroy those who are ripe in wickedness prior to and at the Second Coming of Jesus Christ; 2) We must turn to the Lord in preparation for his Second Coming; 3) Great spiritual and physical blessings will come to the righteous during and after the destructions coupled with the Second Coming.
1 "Book of Mormon", JS-H 1:41