The Book of Joel describes the Day of the Lord through locust swarms, war scenes, and powerful imagery in order for readers to comprehend its magnitude.
No particular mention of Israel's sins is present in the book, but the consequences of the terrible events described are certainly clear: God will one day wipe out all evil and start creation over. But through this destruction also comes eternal protection for those who believe in Him.
Salvation from the Messianic King is once again prophesied, but this time the Holy Spirit is prophesied to come too, causing seemingly ordinary people to have visions, divine dreams, and be filled with God's Spirit. That's exactly what happened during Pentecost to Jesus' disciples after His death and resurrection, ensuring us that the Day of the Lord is even closer at hand today.