That is incorrect. The day of the Lord (which corresponds to the Great Tribulation) is only a portion of the book of Revelation. But Revelation is God's revelation for what will take place from the first coming of Christ until the New Heavens and the New Earth are established. Also "the day of the Lord (LORD)" is being confused with "the Lord's Day" (the first day of the week) by some people who are clueless.
So why do Christians have a hard time interpreting Revelation? Because many who have gone before have muddied the waters, claiming that (1) Revelation is entirely symbolic, (2) we do not need to take the events literally, (3) we do not need to take the events chronologically, and (4) we can play fast and loose with prophecy. Then we have people today claiming that even though Christ (through John) shut the door on any future prophecies. we have prophets walking around today claiming to bring new prophecies to the world.