(Excerpts from our page on the Zorastrian Calendar at Zoroastrian Heritage)

Organization of the Zoroastrian Calendar
The Zoroastrian Fasli calendar is one of the most precise and consistent calendars used in the world today. One, 365-day calendar grid can be used perpetually. The calendar can also serve as a zodiac and seasonal calendar in temperate regions.

(Please refer to the 365-day calendar grid in order to convert a Gregorian/Western calendar date to a Zoroastrian date. Please note that Zorastrian dates have no numbers and are defined by names. These names have associated properties.)

The Zoroastrian calendar is based on a solar year of 365 days. It consists of twelve lunar months of 30 days each, plus five (six in a leap year) intercalary days after the last month. These intercalary days are festival days in preparation for New Year's Day (the spring equinox in the northern hemisphere, or March 21 in the Gregorian calendar). The names of the Zoroastrian month and day confer properties and characteristics which may have astrological significance.

Each month consists of four divisions (which we will call weeks for convenience), beginning on the 1st, the 8th, the 15th and the 23rd day of the month. The first two of seven days and the next two of eight days, giving a total of 30 days. The first day of the month always starts with the first day of the first week and the same day every year falls on the same day of the week.

In Persian, a year is called sol, a month is called mah and a day is called ruz. In the Zoroastrian calendar, the months and days are named (in the western Gregorian calendar, only the months are named).

On this page, will quote extensively from two old Zoroastrian texts: Book 3, Chapter 419 of the Dinkard (Works of the Religion), written in the third century ACE, and Chapter 25 of the Bundahishn (Creation), written after the Dinkard. Both works refer to older sources.

The Seasons, Months, Zodiac & the Zoroastrian Calendar
"The zodiac and the festivals connected with each season are closely connected." (Dinkard 3, chapter 419)

"Spring is the season of the commencement of the year. Spring starts when the sun enters the first degree of Varak (the lamb/ram or Aries) called the halo of the sky and continues for three months [Frawardin (Farvardin), Ardwahisht (Ardibehesht), Hordad (Khordad)] when the sun travels through the constellations of Varak (the lamb or Aries), Tora (the bull or Taurus), and Dopatkar (the two figures or Gemini)." (Dinkard 3, chapter 419 & Bundahishn Chapter 25)

"Summer is the second season of the year and regarded as the season of light. It begins when the sun enters the first degree of the constellation Kalakang (the crab or Cancer), and lasts for three months [Tishtar / Tir, Amurdad (Amordad), Shahrivar] till the sun continues its course through the constellations of Kalakang (the crab or Cancer), Ser (the lion or Leo) and Khusak (Virgo)." (Dinkard 3, chapter 419 & Bundahishn Chapter 25)

"Autumn is the third season. It begins when the Sun arrives at the first degree of Tarazuk (the balance or Libra), known as Star, and lasts for three months [Mihr (Meher), Aban, Adar] till the sun completes its course through of Tarazuk (the balance or Libra), Gazdum (the scorpion or Scorpio), and Nimasp (the centaur or Sagittarius)." (Dinkard 3, chapter 419 & Bundahishn Chapter 25)

"Winter is the last and fourth season. It begins when the sun enters the limit of Vahik (Capricorn) called Dudtora, and lasts for three months [Dae, Vohuman (Bahman), Spandarmad] till the sun travels through Vahik (Capricorn), Dul (the water spout or Aquarius), and Mahik (the fish or Pisces)." (Dinkard 3, chapter 419 & Bundahishn Chapter 25)

The above coincidence between the constellations of the zodiac and the months would only have held true in the year of Varak (the lamb/ram or Aries), that is from approximately 2150-1 BCE. After that the constellations of the zodiac would have gradually regressed through the neighbouring months. Currently, the constellation of Mahik (the fish or Pisces) is at the point of leaving the month of Farvardin, to be replaced by Dul (the water spout or Aquarius). If the age is defined by the thirty-degree house assigned to these constellations then currently we are currently in the Age of Mahik (the fish or Pisces), 1-2150 CE.

What the zodiac-month alignments in the Dinkard and Bundahishn tell us is that the calendar they describe could have been developed around 1 BCE - some two thousand years ago during the Parthian era.

Day and Night
It is always necessary first to count the day and afterwards the night, for first the day goes off, and then the night comes on. (Bundahishn Chapter 25)

The day according to Zoroastrian tradition starts and ends at sunrise - when the first rays of the Sun break over the horizon.

New Year's Day
The Zoroastrian New Year starts at sunrise on the day on the vernal (spring in the northern hemisphere) equinox.

In order to determine the first day of the New Year, two systems can be used to adjust the calendar for fractional days in the solar year: an extra leap year day or astronomical observations. The latter is more accurate and automatically adjusts the calendar.

If the astronomical observation system were to be followed strictly (as it was likely at some point in the past), the insertion of predetermined periodic leap days are unnecessary. Rather, an observatory determines the first day of the year and the calendar is automatically adjusted for the fractional days in a solar year. It is the most accurate method of adjusting the calendar. Since the astronomer-priests would have been maintained a continuous watch of the skies over preceding days, months and years, they would have been able to predict New Year's Day in advance and thereby enabled forward planning of the number of holidays between the end of the last month and the beginning of the first month.

Divisions of the Day
In the seven months of summer the periods (gas) of the days and nights are five, namely, Hawan the period of day-break, Rapithwan the period of midday, Uziran the period of afternoon, Aiwisruthrem the period when the stars appear in the sky until midnight, and Ushahin the period from midnight until the stars become imperceptible. In winter there are four periods, and Hawan extends from daybreak until Uziran (Rapithwan is omitted) while the rest are as previously mentioned. (Bundahishn Chapter 25).

Nowadays, the day is divided into five sections or gahs (also called geh) as follows:

 Havan* gah sunrise to noon dedicated to Mithra
 Rapithwan** gah noon to 3 p.m. dedicated to warmth
 Uziran gah 3 p.m. to sunset dedicated to Apam-Napat
 Aiwisruthrem gah sunset to midnight dedicated to fravashis
 Ushahin gah midnight to sunrise  dedicated to Sarosh

The word 'gah' or 'geh' is taken to mean a division or period of time and a watch. It lends itself to the word gahanbar / gahambar as a division of the year and even to the name of a place as in Atash-Gah and Yazishn-Gah.

In the Avesta, the five divisions of the day are called asnya ratavo or ayara ratavo, and in Middle Persian texts - gahan.

*Havan gah: In the Avestan languages, this watch is called Havani ratu and Havan-ni-Meher (Havan-e Mehr) - the watch when the rays of the rising sun dispel the darkness of the night.

**Rapithwan gah: During the five cool months of the year, i.e., the last five months of the Zoroastrian calendar from Aban to Aspandmard [also see our 365-day calendar grid], the Rapithwan gah is omitted resulting in the number of gahs / watches in a day being reduced from five to four and the Havan gah is extended from sunrise to 3 pm. As we will see in the section below on the Hours (in a day), the days in the cool months are shorter and sunrise occurs later in the morning making this adaptation necessary.

Also see When Does the Zoroastrian Day Start?