Looking at Joel 2:1, Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the LORD cometh, for it is nigh at hand.
For further study - Joel 2:1-10
The prophet Joel introduces to the reader a phrase that is used throughout all of the prophetic passages in God's Word. That phrase, "the day of the Lord", is used to describe a special time in history.
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