text: The calendar had a significant impact on religious observance. It fixed the pantheon of major divinities, and also ensured that their names were uttered often, since at every Zoroastrian act of worship the yazatas of both day and month were invoked. It also clarified the pattern of festivities; for example, Mitrakanna or Mehregan was celebrated on Mithra day of Mithra month, and the Tiri festival (Tiragan) was celebrated on Tiri day of the Tiri month.
  In 538 BC Cyrus the Great (uncertain if he was a Zoroastrian) conquered Babylon and the Babylonian luni-solar calendar came into use for civil purposes.  Cambyses conquered Egypt in 525 BC.  He was accompanied by Darius, a Zoroastrian who became ruler of the Persian empire in 517 BC. The Zoroastrians adopted the wandering Egyptian solar calendar of twelve months of thirty days plus five epagomenal days. As their year began in the spring (with the festival of norouz) the epagomenai were placed just before norouz.
  In Egypt the star Sirius had significance since every 1460 years (the Sothic cycle) its heliacal rising (just before sunrise) marked the Egyptian new year and the inundation of the Nile.   In Persia also the star had significance, since its heliacal rising there also coincided with the coming of the rain.   The fourth Persian month was Tishtrya (Sirius, rain star).   The vernal equinox at Greenwich fell on the first day of the first month from 487 to 483 BC (inclusive).   Adopting S H Taqizadeh's date of 28 March 487 BC for the reform the calendar for that year is as follows: