That again is all up to you and the places and themes you want to play with
It could be that the 'fantastic' has always been there - Perhaps the Romans were hobgoblins and the mongols centaurs, maybe Queen Elizabeth was half-fey, Jack the Ripper a ghoul and Paul Bunyan a real giant.
Or it could have been any event in history - the detonation of the Atomic bomb opened an inter-dimensional portal, perhaps Haileys comet seeded the world with arcane spores, perhaps CERN Collider shifted the orbit of the earth to 'somewhere else". Perhaps the events of Ghostbusters were historic and Gozer the Gozerian now rules New York.