Old Persian calendar

Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the solar observation directly and modified for their beliefs. Days were not named. The months had two or three divisions depending on the phase of the moon. Twelve months of 30 days were named for festivals or activities of the pastoral year. An intercalation month was added periodically to keep the calendar synchronized with the seasons.^[3]

The following table lists the Old Persian months, alongside the approximate Gregorian months and approximate Babylonian lunar months.^[4]

There were four farming festivals, symmetric about maidyoshahem:

Two more festivals were later added, creating the six gahanbar:

Zoroastrian calendar

The first calendars based on Zoroastrian cosmology appeared in the later Achaemenid period (650 to 330 BC). They evolved over the centuries, but month names changed little until now.

The unified Achaemenid Empire required a distinctive Iranian calendar, and one was devised in Egyptian tradition, with 12 months of 30 days, each dedicated to a yazata (Eyzad), and four divisions resembling the Semitic week. Four days per month were dedicated to Ahura Mazda and seven were named after the six Amesha Spentas. Thirteen days were named after Fire, Water, Sun, Moon, Tiri and Geush Urvan (the soul of all animals), Mithra, Sraosha (Soroush, yazata of prayer), Rashnu (the Judge), Fravashi, Bahram (yazata of victory), Raman (Ramesh meaning peace), and Vata, the divinity of the wind. Three were dedicated to the female divinities, Daena (yazata of religion and personified conscious), Ashi (yazata of fortune) and Arshtat (justice). The remaining four were dedicated to Asman (lord of sky or Heaven), Zam (earth), Manthra Spenta (the Bounteous Sacred Word) and Anaghra Raocha (the 'Endless Light' of paradise).

The month names and their modern versions are given in the following table.

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^[5] the calendar for that year is as follows:

The fourth month includes 20 July, the date of the heliacal rising of Sirius. In the first year the people carried on using the old calendar, anticipating festival dates by five days. As each day is named after a god, it is important to observe the celebrations on the right day. Thus the fravasis festival, which in the old calendar was kept between sunset on 30 Spandarmad and sunrise on 1 Frawardin, was now observed throughout the epagomenai. In the second year of the reform, the old 30 Spandarmad was the new 25 Spandarmad, so from then on the festival covered eleven days, up to the new 1 Frawardin. Five days was considered enough for other festivals, however.

In all the lands where the Persian calendar was used the epagomenai were placed at the end of the year. To offset the difference between the agricultural year and the calendar year (the tax-gathering season began after the harvest) the start of the araji (land-tax) year was delayed by one month every 120 years. A Roman historian, Quintus Curtius Rufus, describing a ceremony in 333 BC, writes:

The magi were followed by three hundred and sixty-five young men clad in purple robes, equal in number to the days of a whole year; for the Persians also divided the year into that number of days.^[6]

After the conquests by Alexander the Great and his death, the Persian territories fell to one of his generals, Seleucus (312 BC), starting the Seleucid dynasty of Iran. Based on the Greek tradition, Seleucids introduced the practice of dating by era rather than by the reign of individual kings. Their era became known as that of Alexander, or later the Seleucid era. Since the new rulers were not Zoroastrians, Zoroastrian priests lost their function at the royal courts, and so resented the Seleucids. Although they began dating by eras, they established their own era of Zoroaster.

That was the first serious attempt to determine the dates associated with the prophet Zoroaster's life. Priests had no Zoroastrian historical sources, and so turned to Babylonian archives famous in the ancient world. From these they learned that a great event in Persian history took place 228 years before the era of Alexander. In fact, this was the conquest of Babylon by Cyrus the Great in 539 BC. But the priests misinterpreted this date to be the time the "true faith" was revealed to their prophet, and since Avestan literature indicates that revelation happened when Zoroaster was 30 years old, 568 BC was taken as his year of birth. The date entered written records as the beginning of the era of Zoroaster, and indeed, the Persian Empire. This incorrect date is still mentioned in many current encyclopedias as Zoroaster's birth date.

Modifications by Parthians, Ardashir I, Hormizd I, Yazdgerd III

The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BC, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians. For example, in Achaemenid times the modern Persian month 'Day' was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).

When in April of AD 224 the Parthian dynasty fell and was replaced by the Sasanid, the new king, Ardashir I, abolished the official Babylonian calendar and replaced it with the Zoroastrian. This involved a correction to the places of the gahanbar, which had slipped back in the seasons since they were fixed. These were placed eight months later, as were the epagomenai, the 'Gatha' or 'Gah' days after the ancient Zoroastrian hymns of the same name. Other countries, such as the Armenians and Choresmians, did not accept the change. The new dates were:

In AD 224 the vernal equinox at Greenwich fell at noon on 21 March, which was 22 Shahrewar. Immediately after the reform 21 March corresponded to 27 Shahrewar. Here is the calendar for AD 225–6:

The change caused confusion and was immensely unpopular. The new epagomenai were referred to as "robber days". The people now observed the "Great" nowruz on 6 Frawardin, which was Zoroaster's birthday and corresponded to 1 Frawardin in the old calendar. The new 1 Frawardin was observed as the "lesser" nowruz. Hormizd I (AD 272–273) made the intervening days into festivals as well. In AD 273, the vernal equinox at 0° fell at 05:00 UTC on 21 March.

Yazdegerd I reigned from AD 399–420. In AD 400 the equinox fell about 19 March, which was 9 Aban. According to al-Biruni, in that reign there was a double adjustment of the start of the araji year. The tenth-century astronomer Abu'l-asan Kusyar noted that during the reign of Osrow II (AD 589–628) the sun entered Aries in Adur. This happened throughout his reign. An araji era was introduced dating from AD 621, and the Yazdegerdi era dates from 16 June AD 632, so the Yazdegerdi era is eleven years behind the araji.