Day of the Lord: A biblical phrase prevalent among OT prophets who pointed to a future event or era (not necessarily a single twenty-four-hour day) during which God would visit *judgment on Israel or the world. The NT authors interpreted the phrase in a futuristic sense but saw in Jesus Christ the beginning of the fulfillment of the Day of the Lord. For believers in Christ the Day of the Lord is an anticipation of hope; for unbelievers it holds only judgment leading to *damnation.
Stanley J. Grenz;David Guretzki;Cherith Fee Nordling. Pocket Dictionary of Theological Terms (Kindle Locations 350-353). Kindle Edition.