From Wikipedia article on Ragnarök (emphasis mine):
In Norse mythology, Ragnarök is a series of future events, including a great battle foretold to ultimately result in the death of a number of major figures (including the gods Odin, Thor, Týr, Freyr, Heimdallr, and Loki), the occurrence of various natural disasters, and the subsequent submersion of the world in water. Afterward, the world will resurface anew and fertile, the surviving and returning gods will meet, and the world will be repopulated by two human survivors . Ragnarök is an important event in the Norse canon, and has been the subject of scholarly discourse and theory.
Not wanting to make the question broad, I'm putting it in scope of the Norse mythology and it becomes: If the events of Ragnarok are set in the future, how is it that it is detailed so greatly at present?