Why Do the First Six Persian Months Have 31 Days?
Aktie
The first six months of the modern Persian calendar have 31 days because, together, they form a 186-day first half that closely matches the longer span from the spring equinox to the autumn equinox. The next five months have 30 days, and Esfand has 29 days in an ordinary year or 30 in a leap year. That neat pattern is both practical arithmetic and a reflection of the calendar’s solar design.
It is easy to assume that every solar calendar should divide the year much like the Gregorian calendar. The Persian—or Solar Hijri—calendar does something more regular: long months come first, shorter months follow, and the leap day always goes at the end. Here is how the system works and why its history matters.
Persian month lengths at a glance
| Months | Length | Total |
|---|---|---|
| Farvardin, Ordibehesht, Khordad, Tir, Mordad (Amordad), Shahrivar | 31 days each | 186 days |
| Mehr, Aban, Azar, Dey, Bahman | 30 days each | 150 days |
| Esfand | 29 days; 30 in a leap year | 29 or 30 days |
The arithmetic is unusually tidy:
- Ordinary year: 186 + 150 + 29 = 365 days
- Leap year: 186 + 150 + 30 = 366 days
If the names are unfamiliar, our guide to the meanings of the Persian months from Farvardin to Esfand explains their spellings and cultural roots.
Why are the longer months at the beginning?
The short answer is astronomy. The Persian year starts at the spring equinox, around March 20 or 21. Earth does not move around the Sun at a perfectly constant speed: because its orbit is slightly elliptical, it moves more slowly near aphelion in early July and faster near perihelion in early January. From the Northern Hemisphere’s spring equinox to its autumn equinox therefore takes longer than the autumn-to-spring half of the year.
Calendar researcher Musa Akrami calculates the first of those spans at about 186 days—the exact total produced by six 31-day months. The complementary part of the year is about 179 days in an ordinary year. So the pattern is not an arbitrary block of “extra” summer days. It gives the calendar’s first half the room needed for the longer spring-and-summer portion of Earth’s orbit.
This does not mean each civil month is the exact time the Sun spends in one zodiacal sign. Those astronomical intervals vary. The modern calendar rounds them into fixed month lengths that are easy to print, remember, and use in daily life.
Was this exact structure created by Omar Khayyam?
Not quite. This is where many quick explanations blur two related calendars.
In 1079, the Seljuk ruler Malik-Shah introduced the Jalali reform, associated with a team of astronomers that included Omar Khayyam. Its defining achievement was to restore Nowruz to the spring equinox. In the older astronomical approach, month boundaries could follow the Sun’s movement through zodiacal divisions rather than a permanently fixed sequence of six 31-day months, five 30-day months, and Esfand.
The present civil system was standardized in 1925. It retained the equinox-based New Year and the Solar Hijri year count, revived the familiar Persian month names, and fixed the month lengths into the regular pattern used in Iran today. In other words, the modern calendar inherits the Jalali tradition’s solar foundation, but its exact civil grid is a later standardization.
That distinction matters. Saying “Khayyam invented today’s month lengths” is too simple; saying the modern calendar has no connection to the Jalali reform is equally misleading.
Why does Esfand get the leap day?
Once the first eleven months are fixed, they always contain 336 days: 186 in the first six months plus 150 in the next five. A 365-day year therefore needs 29 days in Esfand. When an extra day is required to keep the next New Year aligned with the spring equinox, Esfand receives a thirtieth day.
This placement has a practical advantage: the correction happens at the end of the year, immediately before 1 Farvardin and Nowruz. The calendar can preserve its clean month pattern while keeping the New Year anchored to spring.
Leap years are also why a Persian date cannot always be converted by adding a single fixed number to the year or memorizing a permanent March date. The exact Nowruz boundary matters. Our article on why Nowruz can fall on March 20 or March 21 explains that boundary in detail, and the Persian date converter is useful when you need a specific Shamsi–Gregorian match.
A simple way to remember the pattern
Think of the year as three blocks:
- Six long months: Farvardin through Shahrivar, all 31 days.
- Five even months: Mehr through Bahman, all 30 days.
- One adjustable month: Esfand, 29 or 30 days.
That is easier to memorize than the alternating pattern of the Gregorian calendar. It also makes date arithmetic straightforward. For example, the last day of Shahrivar is day 186 of the year, and 1 Mehr is day 187. The first day of Esfand arrives after 336 completed days.
Why the pattern still matters outside Iran
For Iranian families in the United States, Canada, and elsewhere, knowing the month lengths is useful for more than formal conversion. It helps with birthdays recorded in Shamsi dates, anniversary planning, school and business deadlines in Iran, and understanding why a family date may shift by a day on a Gregorian calendar from one year to another.
A physical dual-calendar reference can make that bridge easier. Tute’s Persian Hijri Shamsi Desk Calendar 1405 covers March 2026 through February 2027, while the Persian Hijri Shamsi Desk Calendar 1406 continues into the following year. Both are designed around the calendar people actually use, with Persian dates presented alongside a familiar Gregorian frame.
The takeaway
The first six Persian months have 31 days for a reason: six times 31 produces the 186-day first half that closely tracks the longer spring-to-autumn portion of the solar year. Five 30-day months and an adjustable Esfand then complete a year of 365 or 366 days. The result is regular enough for everyday use and precise enough to keep Nowruz tied to the spring equinox.
Image: Planispheric Astrolabe, dated 1065 AH/1654–55 CE, attributed to Mashhad, Iran, by Muhammad Zaman al-Munajjim al-Asturlabi (Iranian, active 1643–1689). Public domain, courtesy of The Metropolitan Museum of Art.
Sources
- Musa Akrami, “The Development of Iranian Calendar: Historical and Astronomical Foundations”—month lengths, the 186-day first half, and the calendar’s astronomical basis.
- M. Heydari-Malayeri, “A Concise Review of the Iranian Calendar”—the vernal-equinox year, intercalation, and historical background.
- Encyclopaedia Iranica, “Nowruz iii. In the Iranian Calendar”—the equinox-based New Year and the 1079 Jalali reform.
- Timeanddate, “Persian Solar Hijri Calendar”—modern calendar structure and official use.