Matthew 24:21 describes the great tribulation. Matthew 24:29 describes the sun and moon darkened AFTER the tribulation of those days. Then, Revelation 6:12 describes the sun and moon darkened, which is the same event as Matthew 24:29. So, in this verse "the great day of his wrath" comes right after the great tribulation. This also demonstrates that the tribulation and the wrath are NOT the same thing. One of them is before the sun and moon are darkened and one of them is after the sun and moon are darkened.