ChãhãrShanbé Suri or literally the ‘RED WEDNESDAY’ falls on the last Tuesday of the year on Iranian calendar (between March 13th to March 20th). It is called suri (sukhr= surkh, meaning ‘red’), because it is one of the many celebrations related to fire. Venerating fire goes back to pre Zoroastrian times in history. Zoroaster brought it into his teachings as a symbol or representation for truth and God.

Another important fire festivity is sadé (= of or relating to sad: this word shares the same etymological root as French ‘cent’ meaning hundred). Sadé falls normally on January 30th of Gregorian calendar.

In 2013 ChãhãrShanbé Suri will fall on Tuesday March 19th. It varies every year since it is the last Tuesday of the year. It depends on when the New Year begins. If the New Year (i.e. the spring equinox moment is Tuesday, ChãhãrShanbé Suri will fall around March 14th. The last month on the Zoroastrian calendar is Spendarma[z]d. In modern Persian it is called Esfand. Zoroastrian calendar is slightly different from the Iranian calendar used today.

All the months’ names are the same in both calendars. Zoroastrian calendar neither has week, nor the name for days of the week like Saturday, Sunday etc. Every month has 30 days (divided into 7+8+7+8). Every day of the month has a name: (day 1): AhurãMazdã (=wise lord= God); (day 2): VohuMan (=good thoughts); (day 3): ArdiBehesht (=best righteousness); (day 4): Shahrivar (=divine kingdom); (day 5): SpandãrMa[z]d (=sacred love); (day 6) KhorDãd (= health and vigor); ا(day 7): Amertãt (=immortality) … (day 30): Anãrãm (endless light). 
Post Islam Iranians adopted a new calendar that was designed by Khayya .ا(famous Iranian poet, mathematician and writer).

The reason was, the Islamic regimes wouldn’t tolerate the Zoroastrian ways. For a while people had to use Arabic calendar which was not on a solar basis and only confused them. Farmers didn’t know how to keep the track of their activity, because unlike any solar calendar Arabic calendar being based on lunar patterns varies from year to year. 1st of Ramadan, as an example, will be in winter and within a decade it would fall on a different season.

So that is why MalekShah (a Seljuk king) ordered Khayyam to create the new calendar, emptying the old one from its Zoroastrian concepts and references.
That is how days of week were added: Sahnbé (=Sabbath, Jewish for Saturday) YekShanbé (=one day after Shanbé =Sunday)… ChãhãrShanbé (four days after Shanbé = Wednesday)… 

Amazing enough ChãhãrShanbé suri is not celebrated on ChãhãrShanbé (Wednesday) but rather on SeShanbé (Tuesday). The reason is, the festivity is called Shab-e ChãhãrShanbé Suri (the eve of ChãhãrShanbé Suri). Shab literally means night but in Persian when someone says shab-e Jom’eh, it means Thursday night and not Friday night. One more thing to clarify about the festivity: In pre-Islam Iran (and among Zoroastrians today) the last 5 days of the year were/are celebrated separately.

As you might have noticed Zoroastrian calendar has 12 months, each month has 30 days (12 X 30 = 360). The 5 days and odd hours left of the year is celebrated as panjé (literally ‘of five’). These five days were dedicated to the dead. Iranians of Pre-Islam era believed and Zoroastrians today still do that the deceased spirits come back to watch how their relatives are doing during their absence. One must try his/her best to look happy.

Thus panjé is celebrated to give the deceased relatives a peace of mind. The event normally is carried out with making bonfires on rooftops. (Do souls watch us from above?) After the invasion of Iran by Arabo-Muslim forces, people who had converted to Islam whether by force or willingly still kept their old beliefs and ways of life. Shab-e ChãhãrShanbé Suri basically is a modified version of Zoroastrian panjé. Even the ‘ãjeel-e ChãhãrShanbé Suri’ or the snack that is consumed on that event is similar to panjé snack, which consists of nuts and dried fruits and is called ‘LORK’--i