Sada, Sadæ/ Çadæ is the most important Iranian winter festival, celebrated by kindling bonfires. Sadæ/ Çadæ falls about 40 days after the winter solstice, namely on the 10th (Ābān) day of the month of Bahman (Vohü  Manö) on or around 30 January.

The etymology of the word Sadæ/Çadæ has been commonly and in my opinion erroneously derived from the numeral ßad (one hundred). The most common explanation of the term is that within the five-month period of the “Great Winter,” the festival fell on the 100th day of the great winter. (Biruni, Āṯār, tr. Sachau, p. 226; Rāzi, pp. 37-38). According to another, less convincing explanation, “one hundred” stands for 50 days plus 50 nights that separate Sadæ/ Çadæ from Vernal equinox or the Iranian New Year.  (Biruni, Āṯār, tr. Sachau, p. 226; Idem, 1954-56, p. 265; Idem, 1983, p. 257; Gardizi, p. 246).
Mary Boyce (1968, pp. 213-15; Idem, 1977, pp. 176-85; Idem, 1983, pp. 800-1) correctly mentions another fire festival still observed by Iranian Zoroastrians, which falls on mid-December, that is one hundred days before Vernal Equinox. This festival is known as  Heerómbá among the Zoroastrians.

In my opinion Sadæ/Çadæ is derived from Avestan Çaræd; cold, Old English cald, German Kalt.  The Sadæ/Çadæ night is considered to be the COLDEST night of the year. It is also called shab-e gazina (the biting night) or shab-e chellæ (the chilliest or 40th night of winter.)

While the custom of lighting fires on the Sada, Sadæ/Çadæ night is forgotten among the Zoroastrians of India, their co-religionists in Iran are still practicing it as a part of their religious observances.

Sada, Sadæ/Çadæ is celebrated by kindling enormous bonfires, drinking wine, and joyous feasting. The bonfires are lit near running water or over an underground water canal (qanāt). This tradition goes back to the ancient belief that the energy/heat of the summer months is transferred into the waters during the winter months. Accordingly, by lighting bonfires close to the waters, the plant life is energized during the most severe freezes. I should add that the festival falls on the 10th day or the day of waters in the 11th month of the Zoroastrian calendar.
Sada, Sadæ/Çadæ continued to be popular even after the Islamic times and it has never completely disappeared in rural areas.  Some traces of a winter fire festival celebrated on the 10th day of Bahman/40th day of winter, or around that date, are survived till present.

The traces of Sada, Sadæ/Çadæ have survived among non-Zoroastrian population of many regions. The custom of kindling Sada, Sadæ/Çadæ fires by non-Zoroastrians was reported for the Ravānduz region in Iraqi Kurdistan and for Šabestar in Azerbaijan (Jashn-e Sada, pp. 32-33), as well as for Kurdish tribes of Mahābād, Kermānšāh, and Qaṣr-e Širin, whose festival of Vehār-i Kurdi (Kurdish Spring) falls on the 45th day of winter (Ayyubiān, pp. 183, 206). A Kurdish fire festival called Tolidān, including some elements of the cult of the prophet Ḵeżr, was celebrated in mid-February by the Kurds of Turkey, Armenia, and Iraq, and it coincided with the Christian Armenian festival of Derendez (Jashn-e Sada, pp. 89-90; cf. Enjavi-Širāzi, II, pp. 120-24).

Sada, Sadæ/Çadæ fires were reported for the cities of Nišāpur, Ferdows, Sabzavār, and Torbat-e Heydariya in Khorasan (Mirniā, pp. 226-27; Rażi, pp. 604-8), for the city of Ḵur in Kavir (Jashn-e Sada, pp. 32-35), and for Lālazār in the province of Kermān.

Happy Sada, Sadæ/Çadæ to you all

ardeshir