Prophecies are very easy to interpret after the event. They are not so easy before the event! Jesus is talking here about the destruction of the Temple, the end of the world and his second coming. It is very interesting here to note that the Temple was actually destroyed in 70AD, the year that Mark is writing his Gospel. Did Mark know about this? Mark wrote in Rome, a long way away and most scholars think he did not know that the Temple was already destroyed. It is still destroyed till this day!!!
His advice for all three events is to be on watch, stay awake, because you never know when the time will come. God sees all things at once. The Prophetic Word sees all things at once. All these things will happen. Do not be in doubt about that.
When these things will happen is not part of the prophecy. We have seen the first part fulfilled. We can be really assured that the other things will happen: Jesus will come again, the world will end. You are the doorkeeper and you are expected to be on guard 24/7.
“And what I say to you, I say to all: Stay awake.”
Jesus comes in a special way at Christmas. Advent is about that. The next two weeks focus on John the Baptist and then Mary and then the Birth of the Christ Child.