Cadä (c is prounounced as in city) is the most important Iranian winter festival, celebrated by kindling fires. Like other seasonal festivals – Noúv-rúz, Tir-gān, and Mehr-gān – it corresponds to Zoroastrian thanksgiving festivals. Sada/Cadä falls 40 days after the winter solstice or yaldaa, namely on the 10th (Ābān) day of the month of Bahman (on or around 30 January in the Gregorian calendar.
Early Iranian Moslem converts condemned the celebration of Sada/Cadä, Noúv-rúz, and Mehr-gān as idolatry for which the Iranians were punished by the Arab invasion (Hamadāni, p. 4; see also Zand, p. 64; Jašn-e Sada, p. 42; Cristoforetti, 2002, pp. 43-44). Yet for Muslim Iranians, who were anxious to retain their cultural identity and historical memory under the Caliphate, these three festivals became a symbol of their glorious past.
Historical and literary sources, confirm the feasting of Sade/Cadä at iranian courts from the Samanid and Ziyarid times up to the Saljuqs (10th-12th centuries) in the areas from Bukhara and Ghazni to Isfahan and Baghdad.
The etymology of the word Sade/Cadä is not clear. Authors generally derive it from the Persian numeral ṣad (one hundred). The most common explanation of the term is that within the five-month period of the “Great Winter,” counted from the first day of the month of Ābān until the end of Esfand, the festival fell on the 100th day of winter, that is the 10th of Bahman (Biruni, Āṯār, tr. Sachau, p. 226; Rāzi, pp. 37-38). According to another, lexplanation, “one hundred” stands for 50 days plus 50 nights that separate Sade/Cadä from the new year on the first day of spring (Biruni, Āṯār, tr. Sachau, p. 226; Idem, 1954-56, p. 265; Idem, 1983, p. 257; Gardizi, p. 246). The term is also put in connection with the legend on the children of the first man or of the first couple (Gayómarṯ, Mašya, and Mašyāna) whose number reached one hundred on that day (Biruni, Āṯār, tr. Sachau, p. 226; Idem, 1954-56, p. 265; Idem, 1983, p. 257; Gardizi, p. 246; Qazvini, p. 80).
I believe that the above explanations are NOT convincing at all, and show only an utter lack of knowledge of the Avestan language, and the Avestan root of many Persian words and terms. Sade/Cadä is most likely an abbreviation of Cardä; Avestan “cared” meaning “cold, frost, winter.” Compare Old English “cald,” Germanic “kaldaz,”  O.S. kald, Old High German kalt; from Proto Indo European base “kál-/cál,” cold, coldness. The fortieth day after Yaldaa or winter solstice is believed to be the time that winter is at its COLDEST. The Sada/Cadä or more accurately Cardä night is considered the coldest night of the year. It was believed that on that night the winter came out from the hell (Biruni, Āṯār, tr. Sachau, p. 226; Qazvini, p. 80) and, according to Biruni, in Karaj it was called šab-e gazina (the biting night). After Sade/Cadä or more accurately Cardä, the weather was supposed to be get warmer.
Iranians celebrated Sade/Cadä by kindling enormous bonfires, drinking wine, and feasting around them. The disappearance of Sade was never complete, and traces of a winter fire festival celebrated on the 10th day of Bahman or January 30th has survived until the present.
The traces of Sada/Cadä or more accurately Cardä have survived among non-Zoroastrian population of many regions. The custom of kindling Sade fires by non-Zoroastrians was reported for the Ravānduz region in Iraqi Kurdistan and for Šabestar in Azerbaijan (Jašn-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. 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 (Jašn-e Sada, pp. 89-90; cf. Enjavi-Širāzi, II, pp. 120-24).
Sade/Cadä or more accurately Cardä 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 (Jašn-e Sada, pp. 32-35), and for Lālazār in the province of Kermān. According to Enjavi-Širāzi (I, pp. 54-57), until modern times a fire festival called Jeljelāni had been held in the city of Naṭanz in west-central Iran on the 17-19 days of Dey, that is some two-three weeks prior to the traditional Sade date, but with many of its characteristics.
At the same time, for rural communities of Iran, the 10th of Bahman or January 30th has kept its importance as a turning point of the winter,when agricultural and pastoral fertility magic is performed: the pastoral Kusa rites in western and central Iran (Enjavi-Širāzi, I, pp. 67-85 and II, pp. 92-115, 170-81; cf. Krasnowolska, 1998, pp. 161-80), orchard magic, fumigation of the fruit trees, hanging stones on their branches, etc. (Enjavi-Širāzi, I, pp. 4, 54-57; Asadiān-Ḵorramābādi et al., p. 214).
While the custom of lighting fires on the Sade night is UNKNOWN to the Zoroastrians Parsis in India, their co-religionists in Iran are still practicing it as a part of their RELIGIOUS observances. Sade or Cardä is the culmination of Zoroastrian the winter solstice and mid-winter/maid yaar festivals. It occurs about 40 days from the eve of the winter solstice, on or about January 30th. The great fire festival of “Caredá/Cardä or “cadä/sade” concludes the COLD SEASON celebrations. For further reading check my previous “Winter Solstice Among Ancient Aryans, Yal-dá, “Maid-Yaar,” Yule-tide and Jól” article.
 
Ardeshir

Bibliography:
ENCYCLOPEDIA IRANICA
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