The Son of Man cometh (ie our untimely death...) when we are judged, for everyman shall give an account of his actions... (for the believer) and judgement (for man is appointed once to die and then the judgement) for the non believer... 

I think that he is asking us to not wait til the last minute to start living for him. Live for him everyday because you do not know when he is coming back. 

He is telling us that no man knows the day nor the hour when the Son of Man is coming and to be yea ready. He (Christ) can come today, tomorrow or even in the next minute. 

He si basically advising us not to be like the virgins in the previous verses, i'e don't hide our light be ready,be making disciples, be doing what he has asked you to do till he comes, 

He is asking us to watch for the return of Christ, and to live our lives as though he could return today, tomorrow, any time and to be prepared.