Which book of the Bible contains prophecies about the day of the Lord?

The book that contains significant prophecies about the "day of the Lord" is indeed Amos. Amos is a prophetic book that emphasizes God's judgment on Israel and the surrounding nations, highlighting themes of social justice, divine vengeance, and eventual restoration. The phrase "day of the Lord" is used to signify a time of reckoning where God will intervene dramatically in human affairs, often associated with both judgment and hope. In Amos, this theme conveys the certainty of God's judgment against Israel for its injustices and unfaithfulness, making it a key concept in the prophecies contained within the book. This prominent usage distinguishes Amos as a critical text for understanding the prophetic portrayal of the "day of the Lord" in the Old Testament.