For a thorough understanding of the time frames referred to as “Great Tribulation” and “Day of the Lord,” please study our free booklet, “The Great Tribulation and the Day of the Lord.”
In short, the Great Tribulation (Matthew 24:21) is followed by the Heavenly Signs (Revelation 6:12-14; Joel 2:30-31; Matthew 24:29) and by the Day of the Lord (Revelation 6:17). The indication is, from Scripture, that the Day of the Lord begins one year before Christ’s return (compare, for example, Isaiah 34:8). But even as the Day of the Lord does not end with Christ’s return, but is ongoing, so the Great Tribulation (which begins approximately 2 1/2 years prior to the Day of the Lord) does not end, either, when the Day of the Lord starts. Rather, it continues until Satan, who causes the Great Tribulation to occur, is removed from his throne.
Christ returns to cut the Great Tribulation short (Matthew 24:21-22). This means, the Great Tribulation is still ongoing — it overlaps for one year, in that sense, with the Day of the Lord. So, all in all, the Great Tribulation lasts approximately 3 1/2 years, but during the last year of the Great Tribulation, God starts to intervene — approximately one year before Christ’s actual return. That the Great Tribulation does not stop when the Day of the Lord begins, can also be seen in Jeremiah 30:7. Although technically speaking about the time of the Day of the Lord, it still refers to Jacob’s trouble, i.e. SATAN’S wrath (Revelation 12:12) or the Great Tribulation that Satan will bring upon the modern descendants of Jacob or the houses of Israel and Judah — as well as the Church of God or spiritual Israel.
The Day of the Lord is the day of GOD’S wrath. Revelation 15:1 says that the seven last plagues COMPLETE God’s wrath. Revelation 16 describes how the angels will pour out the bowls of God’s wrath (compare Revelation 15:7) onto this earth. Please note that these plagues will be poured out BEFORE Christ returns. We read in Revelation 16:14 that the sixth plague PREPARES the way for the kings to come to Jerusalem “for the battle of that great day of God Almighty.” That battle is described in Zechariah 14, and it takes place exactly at the time when Christ returns. After all, the kings will try to fight the returning Christ (Revelation 19:19).
So, we see again that the Great Tribulation lasts, technically, approximately 3 1/2 years, but the last year of that time period overlaps with the beginning of the Day of the LORD — which describes the time when God intervenes in human affairs. The Day of the Lord, then, does not BEGIN with the return of Christ, but when God starts to intervene in human affairs — which is somewhat prior to Christ’s return. Also, as we pointed out before, there is the possibility, of course, that the Great Tribulation might not even last exactly 3 1/2 years, as these days will be cut short (compare Matthew 24:22).