Matt 24, Mark 13, and Luke 21 are all the same story, they are the description that Jesus gave to the apostles about His return and the gathering(rapture) of the church. There is kind of a mystery involved in the whole thing because Jesus described two events at the same time, and they are separated by close to two thousand years. The truth is that Jesus described the destruction of Jerusalem and the Temple, something that happened around 70ad, while He also described the time of His return and the end of the age, something that has not happened yet. Unless we understand that what he said would happen in that generation did in fact happen in that generation we are going to miss it. The destruction of the Temple and the destruction of Jerusalem and the carrying away of the Jewish people all happened around 70ad, in that generation. On the other hand, the world wide persecution of Christians, world war with disease and famine, and the heavens being shaken with Christ return in the sky to gather the church has not yet happened. So part of what Jesus said has already happened, and part of what Jesus said is yet to happen.