be alert

I was reading in the Gospels yesterday and Jesus was talking about how He would come back at an unknown hour.  He uses numerous parables for examples (homeowner and the thief, the bridesmaids and the lamps, etc.) to show how no one could ever know when the time will come.  He goes on and on about how it will be when you least expect it and no one knows the hour.  Then, He tells us to be alert and always ready.  But, how is one to be ready for something when they have no idea when will occur?  How can you be prepared, but not expecting the event?  To throw a further twist in the story, Jesus also tells us that there will be signs to show when He is coming back (the moon and sun will become darkened and the stars will fall).  So there will be signs, but you won't know when it's coming, but you should always be ready for it?  Confused, I am.