Official Calendar

The Persian calendar, Iran's official Calendar is a solar calendar with a starting point that matches that of the Islamic calendar. Its origin can be traced back to the 11th century when a group of astronomers (including the well-known poet Omar Khayyam) created what is known as the Jalaali calendar. However, a number of changes have been made to the calendar since then.
The current calendar has been used in Iran since 1925 and in Afghanistan since 1957. However, Afghanistan used the Islamic calendar in the years 1999-2002(Taliban period).

The names and lengths of the 12 months that comprise the Persian year are:

S P R I N G 1 Farvardin 20 Mar.-19 Apr. 31 days 2 Ordibehesht 20 Apr.-20 May. 31 days 3 Khordad 21 May.-20 Jun. 31 days F A L L 7 Mehr 22 Sep.-21 Oct. 30 days 8 Aban 22 Oct.-20 Nov. 30 days 9 Azar 21 Nov.-20 Dec. 30 days

S U M M E R 4 Tir 21 Jun.-21 Ju. 31 days 5 Mordad 22 Jul.-21 Aug. 31 days 6 Shahrivar 22 Aug.-21 Sep. 31 days W I N T E R 10 Day 21 Dec..-19 Jan. 30 days 11 Bahman 20 Jan.-18 Feb. 30 days 12 Esfand 19 Feb.-20 Mar.. 29/30 days

The month of Esfand has 29 days in an ordinary year, 30 days in a leap year.
 
The Persian year starts at vernal equinox. If the astronomical vernal equinox falls before noon (Tehran true time) on a particular day, then that day is the first day of the year. If the astronomical vernal equinox falls after noon, the following day is the first day of the year.
As in the Islamic calendar, years are counted since Prophet Mohammed's emigration to Medina in AD 622. At vernal equinox of that year, AP 1 started (AP = Anno Persico/Anno Persarum = Persian year).
Note that contrary to the Islamic calendar, the Persian calendar counts solar years. In the year AD 2004 we have therefore witnessed the start of Persian year 1388, but the start of Islamic year 1425.
There is a leap year our of each 4 years. (it's more complex and there are some exceptions.)

CALENDAR CONVERTOR

Official Holidays :
Friday is the official weekend, although Thursday are closed for many businesses.
Iran's official holidays are being extracted from  two calendar systems :
1. National Official Calendar (Hijri-Shamsi) which is a Solar Calendar (1 year = 365 days) and is easily comparable to Gregorian (international) calendar.
2. Islamic Calendar (Hijri-Qamari) which is a Lunar Calendar (1 year=355 days) which causes the religious holidays move along the solar year.
Hence, as a rule of thumb, national Iranian holidays are fixed on either Iranian National Holiday or International calendar, but the religious holidays must be checked with the official calendars each year.
The following table shows the official holidays for Iranian year 1390 which coincides 21 Mar. 2011-19 Mar. 2012.