It is thought that it would be due to a pole shift, there is evidence that it used to occur in the past and it caused extinction of dinosaurs and various animal species. Some say it occurs periodically with regularity, some say it's not regular. There is evidence of weakening polarity of magnetic field of the earth now and it sure is going to happen sometime in the future, but it seems to me improbable that it would happen so suddenly on one day and that it can be calculated so precisely, much less by some ancient culture and so far in advance. But I might be wrong.