Sadeh is a mid-winter celebration observed by Zoroastrians. It is a deeply religious festival. Sadeh has a complex history, and two different traditions are to be noted. According to the Fasli calendar, this would place it on Dec 11.
People begin to gather an hour before sunset, a spot near a stream seems to be preferred.
Although it is mentioned in the Qissa-i Sanjan , Sadeh does not seem to have much significance among modern Parsi Zoroastrians. He says the fire is to symbolize the approach of winter which necessitates the kindling of fires.