Sadeh is a mid-winter celebration observed by Zoroastrians. It includes preparing a large bonfire and is therefore also known as Adur-Jashan (Feast of fire). The bonfire is to drive back the winter in defiance of Ahriman (satan). It is a deeply religious festival.
Sadeh has a complex history, and two different traditions are to be noted.
By Yazd tradition, it is observed on Ashtad ruz, Adur Mah. This is the 100th ("sadeh" in Persian) day before Noruz. According to the Fasli calendar, this would place it on Dec 11. The other day, observed by Kermani Zoroastrians, is Aban ruz, Vohuman mah (hundredth day after the gahambar of Ayathrima, held to be the beginning of winter) = January 24.
The chief preparation is the gathering of wood, and everyone in the community is expected to contribute:
Shāx-e shāx-e (h)armanī 'A branch, a branch...! Har kas shāx-e be-dehad Whoever gives a branch, odā murād-esh be-dehad! May God grant his wish! Har kas shāx-e na-dehad, Whoever does not give a branch, Khodā murād-esh na-dehad! May God not grant his wish!'
(Perhaps there is some connection here with the custom of wishing on candles at birthdays.)
People begin to gather an hour before sunset, a spot near a stream seems to be preferred.
The lighting of the fire is properly preceeded by an Afrinagan-e Do Dahman, a ceremony of blessing for the whole community, and Atash Niyayesh (fire litany).
Although it is mentioned in the Qissa-i Sanjan, Sadeh does not seem to have much significance among modern Parsi Zoroastrians. The prolific Parsee writer Dr. J.J. Modi does makes a very brief note of it in his Religious Ceremonies and Customs of the Parsees (1922, p. 464). He gives the date as ruz Aban, mah Deh (= December 25 by Fasli reckoning!) He says the fire is to symbolize the approach of winter which necessitates the kindling of fires.
Sadeh is discussed at length by Mary Boyce in A Persian
Stronghold of Zoroastrianism, chapter 7, and her article
'the 2 dates of the feast of Sada' (Farhang-e Iran-Zamin,
xxi. 4 (1976), 25-40).