The Iranian calendar is based on the movement of the sun. The present calendar was formulated by a number of great scholars, notably the famous mathematician and poet, Omar Khayyam. It begins with the journey (Hegira) of the holy Prophet Mohammad (S) from Mecca to Medina in 621 AD, i.e. the year 621 AD is the first year of the Iranian calendar. Consequently, the year 2001 AD coincides with the Iranian year 1380 SH (Shamsi [solar] Hejri or Hrgira).
The Iranian year begins on the first day of spring; i.e. it is the first day of the first month of the first season of the Iranian year, when the length of the day and night is equal. The Iranian year has four seasons and every season has three months.
The first six months of the Iranian year have 31 days each. The next five months are 30 days long and the last month has 29 days except that every four years there is a leap year when the last month is 30 days long.

The sequence of the months and days, and the dates they coincide with, are as given below:

Spring
Farvardin: 21 March-20 April
Ordibehesht: 21 April-21 May
Khordad: 22 May-21 June

Summer
Tir: 22 June-22 July
Mordad: 23 July-22 August
Shahrivar: 23 August-22 September

Autumn
Mehr: 23 September-22 October
Aban: 23 October-21 November
Azar: 22 November-21 December

Winter
Day: 22 December-20 January
Bahman: 21 January-19 February
Esfand: 20 February-20 March

One of the characteristics of the Iranian calendar is that the first day of the year or the New Year's Day (Norooz or New Day) coincides with the first day of spring, when nature begins to wake up after its winter sleep. It is for this reason that the UN has named the Iranian New Year's Day the Day of the Earth. The beginning of the year is celebrated gloriously and the celebrations are called Norooz, a ceremony that has a history of some thousands of years. The festivities last 13 days and normally there is a countrywide bank holiday during the first 4-5 days and on the 13th day.
During the New Year holidays most of the people leave their homes to travel in the rest of the country, as is the custom. They go to visit the more attractive and climatically pleasing parts of the country.
"Sofreh Haftseen" or the tablecloth with seven Ss, is a custom that is seriously observed by all Iranians. Before the New Year begins a tablecloth is spread on the floor or a table and seven objects are placed on it whose names begin with an "S". In addition a volume of the Holy Koran and a few gold fish in a bowl of water are also placed on the cloth..
On the 13th day of the new year which is the last day of celebration and is known as "Sizdeh-be-dar" (out on or with the 13th) every body goes to an outing, a picnics in the country, in a park, by a river... somewhere closer to nature. They spend a joyful day in the midst of nature and dispel the bad omen of the figure 13.
Norooz is the most important Iranian festivity. Before this festivity begins and during the days of its duration Iranians extravagantly celebrate the coming of the new, the revival of nature with spring. The 12th day of Farvardin (the 1st month of the Iranian year) is known as the Day of the Islamic Republic because it was on this day in 1979 (2nd April 1979) that Iranians participated in a referendum and almost unanimously voted for an Islamic Republic.
The 14th of Khordad (4th June) is the day Imam Khomeini (RH) passed away and is an official holiday. The 15th of Khordad (5th June) is the day of the uprising of the citizens of Qom who in 1963, led by Imam Khomeini (RH), organized demonstrations against the Shah's regime in which a number of citizens lost their lives. This too is an official holiday.
The 22nd of Bahman, the day of the victory of the Islamic Revolution in 1979 (11th February) and the 29th of Esfand (20th March), the day of the nationalization of the oil industry, are also official holidays.
From 12 to 22 of Bahman (1st-11th of February) celebrations are held as the Ten Days of Fajr on the occasion of the victory of the Islamic Republic. It was on the 12th of Bahman 1979 that Imam Khomeini (RH) returned to Iran, from exile in Paris, and was received by millions of happy people. Ten days later the Islamic Revolution's victory was established. During the ten days of this celebration many festivals are held. In particular, the country's most important film festival and a number of artistic and sports contests and tournaments are held during these ten days.
But many festivities and occasions of official mourning in Iran are held according to the Islamic calendar which runs on the movement of the moon round the earth and the earth round the sun observed as changes in the shape of the moon. This calendar has a 355 or 354-day year, i.e. it is 11 days shorter than the solar year, on average. So, many Islamic occasions are held on different days with respect to the Iranian solar year. The Islamic year has 12 months as follows: 1- Moharram, 2- Safar, 3- Rabi-ol-Awwal, 4- Rabi-ol-Thani, 5- Jamadi-ol-Awwal, 6- Jamadi-ol-Thani, 7-Rajab, 8- Shaban, 9- Ramazan, 10- Shawwal, 11- Zighadeh, 12- Zihajjeh.
Because of the 11-day difference, the Islamic calendar does not coincide with the Iranian calendar. So, the Ashura, which is the day of martyrdom of Imam Hossein (AS), the third Imam of the Shias, and which is held on the 10th of Moharram of each year coincided with 27th of Farvardin of 1379 (16 April 2000). In 1380 it coincided with the 16th Farvardin 1380 (2001) and in 1381 it will coincide with the 5th of Farvardin (2002).

Iranians' most important annual occasions of feast or mourning, which are also official holidays, are as follows:
9 th & 10 th Moharram: mourning for the martyrdom of the third Imam of Shias, Imam Hossein (AS), and his entourage.
20 th Safar: Arbain, the 40th day after the martyrdom of Imam Hossein.
28 th Safar: the death of the holy Prophet Mohammad (peace be upon him).
30tth Safar:the: death of Imam Reza, the eight Imams.
17 th Rabi-ol-Awwal: the birthday of the Prophet and the sixth Imam of Shias, Imam Jafar Sadegh.
3rd Jamadi-ol-Thani: the death of Hazrat Fatemeh, the daughter of the Holy Prophet.
13 th Rajab: the birthday of Imam Ali (AS)
27th Rajab: the day the Holy Prophet was informed of his prophethood.
15 th Shaban: the birthday of Hazrat Mahdi (AS), the 12th and last Imam of Shias.
21 th Ramazan: the martyrdom of the 1st Imam, Hazrat Ali (AS).
1st Shawwal: Eede Fetre, the end of Ramazan (the month of fasting).
25 th Shawwal: the martyrdom of Imam Jafar Sadegh.
10 th Zihajjeh: Eede Ghorban.
18 th Zihajjeh: Eede Ghadire Khom.

The months of Moharram and Ramazan are the two most important months for Shias, and in the lives of Iranians. During Moharram, Iranians spend sorrowful days mourning the martyrdom of Imam Hossein (AS), the son of Imam Ali (AS) and the grandson of the Prophet.
During Moharram most Shias dress in black, gather in mosques and mourn Imam Hossein and all that he suffered. The mourning reaches a pinnacle on the 9th and 10th of Moharram (the martyrdom took place on the 10th).
Ramazan is the month of fasting. All Moslems refrain from eating, drinking and smoking during the daytime. They can eat at sunset. After eating they go to mosques to mourn Imam Ali (AS), the first Imam of Shias, who was struck with sword while in prayer, on the 19th of Ramazan, and passed away on the 21st.
Those who visit Iran during Ramazan must remember that Moslems are very sensitive about their religious principles. Visitors must refrain from eating, drinking or smoking in public during Ramazan. But they may do so in the hotels, airports etc, where they will be given excellent reception.