The Ancients of Iran chose to celebrate the beginning of each season, as it related to the inteplay of light and darkness, and the position of the Earth in the larger scheme of things in the Universe. This year Spring Equinox or Nowrooz will be celebrated at exactly 11:43:39 GMT today all over the world.
Ancient dualism or what they call Mithraic is part of the precession of Equinoxes as explained in this video.
