"This phrase occurs five times in Joel and is the dominant theme (1:15; 2:1; 2:11; 2:31; 3:14). Six other prophets also use it: Isaiah (13:6, 9), Ezekiel (13:5; 30:3), Amos (5:18, 20), Obadiah (15), Zephaniah (1:7, 14) and Malachi (4:5); and an equivalent expression occurs in Zec 14:1. Sometimes abbreviated as 'that day,' the term often refers to the decisive intervention of God in history, such as through the invasion of locusts in Joel or at the battle of Carchemish, 605 B.C. (see Jer 46:2, 10). It can also refer to Christ's coming to consummate history (see Mal 4:5; Mt 11:24; 1 Co 5:5; 2 Co 1:14; 1 Th 5:2; 2 Pe 3:10). When the term is not used for divine judgments in the midst of history, it refers to the final day of the Lord, which generally has two aspects: (1) God's triumph over and punishment of his enemies and (2) his granting of rest (security) and blessing to his people." (THE NIV STUDY BIBLE NOTES, Joel 1:15)
