Fun facts
Surprising calendar facts
Ten days that vanished, a calendar that repeats itself every 400 years, and a New Year that arrives at a different minute every spring. Some of our favourite facts about the calendars we live by.
The Persian calendar
Spring and summer get a bonus week
The first six Persian months have 31 days for a real reason: Earth moves more slowly near its farthest point from the Sun, in early July, so the Sun spends about 186 days in the northern half of the sky and only about 179 in the southern half. Six months of 31 days make exactly 186.
Leap years follow the sky
There is no fixed leap-year rule: a year is a leap year when the March equinox makes it one. The pattern that results has about eight leap years every 33 years, usually four years apart, with an occasional five-year gap — 1403 to 1408 is one.
Persian weekdays are numbers
The week starts on Saturday, shanbeh. Sunday is yekshanbeh, “one after Saturday”; Monday is doshanbeh, “two”; and so on to Thursday, panjshanbeh, “five”. Only Friday, jomeh, has a name of its own.
The longest night has a name
The night before 1 Dey — the winter solstice — is Shab-e Yalda. Families stay up late, eat pomegranates and watermelon, and read the poetry of Hafez to see out the year’s darkest night.
The year Iran jumped ahead 1,180 years
In 1976 Iran briefly replaced the Hijri year count with an imperial era counted from Cyrus the Great. Overnight, 1355 became 2535. The change was reversed in 1978.
A poet helped build a calendar
The Jalali calendar, adopted in 1079 under the Seljuk sultan Malik-Shah, was prepared by a committee of astronomers that included Omar Khayyam — better known abroad for his quatrains, the Rubaiyat.
The Gregorian calendar
Ten days that never happened
To realign the calendar with the seasons, Thursday 4 October 1582 was followed by Friday 15 October in the countries that adopted the Gregorian reform first. Britain waited until 1752 — and had to skip eleven days.
The calendar repeats every 400 years
Four hundred Gregorian years contain 146,097 days — exactly 20,871 weeks. So every 400 years the calendar, weekdays and all, starts over: 2026 looks exactly like 1626 and 2426.
Friday the 13th is the most common 13th
Over a full 400-year cycle, the 13th of the month falls on a Friday 688 times — slightly more often than on any other weekday. We checked: our own calendar engine counts the same.
September means “seventh”
September, October, November and December come from the Latin for seven, eight, nine and ten. The early Roman year began in March — just like the Persian year, which begins at the March equinox.
February’s missing days weren’t stolen
February was the last month of the early Roman year and got what was left over. The popular story that Augustus took a day from it for August is a medieval invention, usually traced to the 13th-century scholar Johannes de Sacrobosco.
There was no year zero
Gregorian years jump straight from 1 BCE to 1 CE. The Persian calendar has no year zero either: its first year is year 1.
Around the world
Two Hijri calendars, 43 years apart
The solar and lunar Hijri calendars both start in 622, but a lunar year is about 11 days shorter. The lunar count gains a year roughly every 32½ solar years — which is why, in November 2026, it is 1448 in the lunar calendar but 1405 in the solar one.
Plenty of New Year’s Days
The Persian New Year arrives at the March equinox. Chinese New Year falls between 21 January and 20 February, Rosh Hashanah in September or October, the Ethiopian New Year on 11 or 12 September — and the Islamic New Year moves about 11 days earlier each Gregorian year.
One day, many names
Every calendar is just a way of naming days. Astronomers sidestep the names entirely and number every day since 4713 BCE — the Julian Day Number. ShamsiDate uses that count internally: 1 November 2026 is day 2,461,346.
Is it a leap year?
Type any year — Persian or Gregorian — to see whether it is a leap year in both calendars.
Want to see more? Put any date into the converter, or explore how the Persian calendar and the Gregorian calendar work.
Sources
- Kazimierz M. Borkowski, “The Persian calendar for 3000 years”, Earth, Moon and Planets 74 (1996): 223–230. doi:10.1007/BF00055188
- Encyclopaedia Britannica, “The Gregorian calendar”. britannica.com
- Calendar (New Style) Act 1750. legislation.gov.uk
- MacTutor History of Mathematics, “Omar Khayyam”. mathshistory.st-andrews.ac.uk
- “Shahanshahi calendar”, Wikipedia. en.wikipedia.org
- “Friday the 13th”, Wikipedia (frequency over the 400-year cycle). en.wikipedia.org
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