The Day of the Lord texts of Malachi (Mal 3:2, 7; 4:5) demonstrate that he seems to allude to Joel's Day of the Lord (2:11, 13, 31 [MT 3:4]). Malachi's Day of the Lord seems to have a strong inner-biblical relationship with the Day of the Lord motif of Joel. A significant interpretive loss is committed when allusion is recognized in the source text but ignored and not explored. Thus, the passages themselves call for an inner-biblical allusion study. In addition, the interpretive significance of Joel's Day of the Lord in Malachi has not been investigated comprehensively as the review of literature and intertextual and inner-biblical studies on Malachi validated. Thus, these are the interrelated questions: (a) "What are the criteria to verify the inner-biblical connection between Malachi and Joel on the Day of the Lord?" (b) "How did Malachi use Joel's Day of the Lord motif?" (c) "How did Malachi develop the Day of the Lord theme in his book?" There are three purposes of this study. The first is to identify objective criteria to verify the inner-biblical connection between Malachi and Joel on the Day of the Lord. The second is to determine how Malachi used Joel's Day of the Lord motif. The last is to ascertain how Malachi developed the Day of the Lord theme in his book.