The expression "day of the Lord" or sometimes simply "that day" was used in the OT for a decisive intervention of God into history. Although occasionally used for historical events (e.g., the destruction of Jerusalem in 586 B.C., Jeremiah 46:10; Ezekiel 7:19; 13:1-9), it was used primarily for the great eschatological intervention of God in which He will overthrow His enemies and establish His rule. Because in the NT these same events are related to the future coming of Christ, the day is there often related to Him (e.g., "the day of our Lord Jesus Christ," 1 Corinthians 1:8; 5:5). The day is characterized both by judgment and vengeance on the wicked (in Israel and the nations) and salvation and deliverance for the people of God. The salvation is portrayed in the OT primarily as the restoration of Israel. But the accompanying teaching of divine salvation extending to the Gentiles as well as the NT teaching concerning the blessing of the church in that day makes it evident that the salvation is for the repentant Gentiles as well. The salvation of the day of the Lord culminates with the transformation of all creation in the making of a new heavens and earth. (See also END TIMES).

This topic is from the Lockman Foundation.