The Great Tribulation
The great tribulation is divided into two equal periods of 1260 days each. During this time there will be the judgement of the Jews Ez. 20:34-38, 22:19-22; Jer. 30:4 -7; dan. 12:1. The day of the Anti-Christ when the Anti-Christ is accepted John 5:43 The Kingdom is counterfeited.
The happenings at this time introduce the Day of The Lord (Rev. 1:10; Isa. 2:11-17; Joel; Matt. 24, etc.). It is the day of the Lord’s wrath (Rev. 6:7). In the Old Testament it is called “Jacob’s Trouble” (Jer. 30:7). In the New Testament, it’s better known as “The Great Tribulation” (Matt. 24:21)
You may want to listen to the study "Four Days in Scripture" which explains more about the Day of the Lord.