Deja Vu is caused by one side of the brain recieving and processing information a microsecond before the other side of the brain; then when the two sides "talk" to each other, one tells the other it has already happened. All this happens too fast for the event to be stored into memory before the feeling of Deja Vu, so you know/think it happened to you before, but you can never remember exactly when it was, because the event was merely a microsecond earlier and not yet stored in the memory.
As far as doing something the day after you dreamed you did it; that is most likely subconcious motives getting you to take a course of action ensuring the event in your dream would come true, if you wanted it to... the power of the mind is insane.
