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In pre-#Avestan Iran, the #calendar consisted of 12 months of 30 days each. Since this is obviously, clearly less than the actual length of a year, every six years, they would postpone New Year's Day by a month.

This would be like if instead of having leap years, we waited ≈120 years and declared that New Year's Day now fell on 1 February, then after another 120 years it fell on 1 March, and so on.

2/?

Circumstantial evidence suggests that #Egypt may have used the same calendar in the distant past, but at some point the Egyptians took the eminently sensible step of adding five extra days to the end of the year.

Iran conquered Egypt in the 9th century BH (before Hejirah) and Egyptian ideas flowed into Iran. One of these ideas was the calendar, and if it was good enough for the people who built the #pyramids, it was good enough for them.

3/?

The Egyptians were aware that the tropical year was slightly longer than 365 days, but didn't adopt a leap year system because only gods could create new days.

Iran had a long tradition of leap year adjustments, and had no problem doing so. At first, they continued as they had; every 120 years, the five extra days were pushed back by a month. At some point, they stopped doing that, and instead just repeated the last month every year; this meant each year had the same months.

4/?

This calendar spread to #Armenia at the time a satrapy of Iran. It was used for centuries, albeit without the leap year rule (because timing things over centuries is hard). Armenian mythology has it that King Hayk introduced the calendar to comemorate his victory over the Babylonians.

But now we need to take a detour and talk about the Arab calendar.

5/?

Al-Biruni, in describing the Indian calendar, says the Hindus measure time the same was as the Jews and the pagan Arabs. Al-Biruni did make some mistakes, but he always strove to accurately describe the beliefs of non-Muslims. As such, most historians believe that the pre-Islamic Arab calendar was lunisolar and occasionally added an extra month called Nasi'

A minority view holds that the Arabs used an Iranian-style cyclic calendar, and Nasi' referred to delaying New Year's Day.

6/?

Just three years after the fall of Mecca, Arab Muslims marched into an Iran devastated by a recent civil war. This, coupled with the fact that Islam has a lot in common with Zoroastrianism, meant that Islam gradually but steadily took hold as Iran's dominant religion. Many Zoroastrians fled to India, while some remained and are today a protected minority.

8/?

Just as Roman culture shaped Christianity, so Iranian culture shaped Islam. It is remarked that Iran lasted millennia without needing Arabs, while after the conquest, Arabs couldn't last a day without Iranians.

That even applied to the calendars. The Arabs were using the purely #moon-based Lunar Hijri calendar, which is OK for desert nomads, but as the civil servants impressed upon #Caliph #Umar, a sedentary agricultural society needs a calendar that tracks the #sun.

9/?

The result was that the existing Zoroastrian calendar, of 365 days, was permitted to be used for exclusively secular purposes, while holidays and religious observances would be set by the Lunar Hijri calendar. Iranian Zoroastrians eventually stopped adding extra months, but Indian Zoroastrians did it once more.

Fast forward to 457 AH, and Malik-Shah entrusts the great scholar, astronomer, and mathematician #OmarKhayyam with improving the calendar.

10/?

Oamr Khayyam's initial proposal defined the day of the northward equinox as Nowruz, or New Year's Day. Each month lasted for as long as the sun took to traverse 1/12 of either the ecliptic or the zodiac (the information I have is unclear).

Since this required either direct observation or a whole lot of tedious and complicated calculations, it was less than ideal. Khayyam came up with a more reasonable proposal—the year still started on the equinox, but the months had fixed lengths.

11/?

Omar Khayyam also came up with a leap year algorithm:
① Divide the year number by 33
② If the remainder is >1 and is 1 greater than an integer multiple of 4, it is a leap year.

This results in 8 leap years in 99 years, and is rather more accurate than the Gregorian calendar, introduced nearly 500 years later.

Khayyam's student, Ahmad Birashk, came up with a much more complex system based on cycles of 2820 years. It is on average more accurate, but the equinox drifts more.

12/?

The observational version of Khayyam's calendar is known as the Solar Hijri calendar, and is today the official calendar used in Iran and Afghanistan.

Some #Zoroastrians use a variant, which also ties Nowruz to the equinox but retains their traditional month structure. This is known as the Fasli (seasonal) calendar. A few conservative Zoroastrians reject it, thinking it is too Muslim.

Yes, the same calendar that Muslims originally thought was too Zoroastrian.

13/?

Some other Zoroastrians add an extra day every four years, and some celebrate Nowruz on 21 March by the Gregorian calendar. And some, mostly in India, still hold to a system that does not acknowledge leap years at all.

Some Indian Zoroastrians still celebrate Nowruz a month later, since they continued to add extra months after the practice ended in Iran. Others decided to adopt the schedule used in Iran, as that is where their religion originated.

14/?

Nowruz today is a secular national holiday in Iran, and a religious observance for Zoroastrians, centered around visiting family and friends. While the day is the one whose midnight falls closest to the instant of equinox, people perform the main celebration at the actul equinox.

Traditionally, this would have been marked by ringing bells; nowadays, it is given by TV and radio accouncements.

So that's the story of Nowruz. Happy Nowruz to those who celebrate.

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Addendum: As you can tell from this thread, even something as seemingly banal and inoffensive as a calendar holds cultural power. #Kurds, for example, use a variant of the Solar Hijri calendar which starts in 1234 BH, year of the Battle of Nineveh, which marked the end of Assyrian hegemony over the Middle East and the ascension of Kurdish Med as the dominant regional power. This calendar is illegal in Türkiye and Syria because it is strongly associated with #KurdishNationalism.

In 1353 AH, Mohammed Reza Pahlavi, the last king of Iran, changed the calendar era from the year of the Hijra to the first year of the reign of Cyrus the Great, to appeal to Iranian nationalists. After the Islamic Revolution in 1358 AH, they reverted it to dating from the Hijra to reinforce that Iran was (and to this day is) a Muslim country.

@Infrapink
Just reading your calendar stuff which I find really really interesting. Wondering how your interest in calendars even started. They are actually quiet weird things I think. Why do we even need them I often think

@Geordielass Aww, thanks, so glad you enjoyed it!

I think it originally sparked when I came across this page, which has info on several: https://www.fourmilab.ch/documents/calendar/

I'd wanted to learn to program for a while, and made that my project when I had some COVID-related downtime. After working through a book, writing a calendar converted seemed like a doable project, and that sent me down several rabbit holes.