In ancient Iran, before the Arab invasion, the Zoroastrians used two systems of time-reckoning - Sal Oshmurtik and Sal Vihezakik. These were the year of ordinary reckoningand the leap year respectively. Since the first had a fixed span of 365 days, Parsis in Iran as well as in India decided long ago to adhere to it rather than the second obsolete system. The Zoroastrian calendar was connected with seasons and solstices in the distant past.
The Parsi calendar follows a luni-solar system of reckoning, which means that it takes into account the movement of both the sun and moon while calculating the calendar. The year is divided into 12 months of 30 days each. At the end of the twelvth month, five more days are added to the year, when the Muktad ceremonies are held. The Parsi year thus consists of 365 days.
Each of the three Parsi sects follow their own calendar. The three calendars are similar, with minor differences.
For instance, although all three calendar years have 365 days, they all have a different date to mark the new year. The Fasli sect maintains 21st March as their Navroz or new year and adjusts the calendar by having one extra day during the Muktad in the leap year. The other two calendars do not make this adjustment, and thus their Navroz goes back one day every four years. The Kadmi calendar is exactly the same as the Shahenshai, except that it is one month ahead of it.
The names of the months and days are the same in all three calendars. The days of the month (roj) are named after the yaztas or guardian angels. These are spiritual beings who preside over the spiritual and physical worlds. Seven of these are amesha aspentasor archangels. The remaining are their co-workers or hamakaras. The first seven days of the month are named after the amesha spentas.
The days of the month are: 1. Hormuzd, 2. Bahman, 3.Ardibehest, 4. Sheherevar, 5. Ashpandarmad, 6. Khordad, 7. Amardad, 8. Daepadar, 9. Adar, 10. Ava, 11. Khorshed, 12. Mohor, 13. Tir, 14. Ghosh, 15. Daepmeher, 16. Meher,17. Sarosh, 18. Rashne, 19. Farvardin, 20. Behram, 21. Ram, 22. Govad, 23. Daepdin, 24. Din, 25. Ashisvangh, 26. Ashtad, 27. Asman, 28. Zamyad, 29. Marespand, 30. Aneran.
The last of the five days of the year are called Gatha days and are part of the Muktad ceremonies. One of the five gathas are recited on each of these days.
The names of the gatha days are: 1. Ahunavad, 2. Ushtavad, 3. Spentamad, 4. Vohuxathra, 5. Vahishtoist
The names of the months are the same for the Kadmi and Shahenshai Parsis. However the dates differ. According to the Kadmis, Farvardin corresponds to September - October, and so on. The Shahenshais take Farvardin to correspond to August - September, and so on.