The Day of the Lord (Yom YHWH) is the prophetic concept of a decisive divine intervention in history — a day of cosmic judgment, atonement, and restoration. Amos introduces it (~760 BC) with a shocking reversal: the Israelites expect the Day of the Lord to be a day of vindication against their enemies; Amos declares it will be a day of judgment against Israel itself (‘darkness, not light’ — Amos 5:18). Isaiah, Joel, Zephaniah, and Malachi develop the concept: universal judgment on all nations, purification of Israel, cosmic signs, and ultimately the establishment of divine sovereignty. The Day of the Lord is the eschatological dimension of Abrahamic atonement theology — the final cosmic reckoning toward which all history moves. It shapes: Jewish High Holiday theology (Rosh Hashanah as the Day of Judgment, Yom Kippur as its resolution); Christian eschatology (the Last Judgment, Second Coming); and Islamic theology of the Day of Resurrection (Yawm al-Qiyama) — the most prominent theme in the Quran after tawhid.