Like all other Daevas (Zoroastrian demons), Apaosha is the adversary of a certain Yazata , in this case Tishtrya , representative of the rain. Both in the Avesta and in the texts of the Zoroastrian tradition (8th – 10th centuries) Apaosha is depicted as a bald, black steed, which starts with the heliacal rise (sunrise) of the constellation Sirius (Av. Tishtrya ) the rainy season tried to prevent. In tradition (but only hinted at in the Avesta), the upcoming change of season is described as an annual 40-day battle between the black horse (Apaosha) and the white gray horse (Tishtrya).
According to tradition, a similar fight takes place in winter; in this Apaosha tries (as frost) to prevent the thawing of the earth. This tradition is also related to the rising of the dog star, here the acronymic rising (rising at sunset), which took place 2,500 years ago on 21-26. December took place (see Cambridge History of Iran , Vol. II, p. 787). To this day, the 5th Gahambar festival - one of the seven holiest days in the Zoroastrian calendar - takes place on December 21st. In Islamic Iran, the 5th Gahambar festival was reinterpreted as a festival for the winter solstice.
Like all conflicts between good and evil, the one between Apaosha and Tishtrya at the end of time ( Frashokereti , the “Final Renewal”) should be settled in favor of the good, and Apaosha will then finally succumb to the Tishtyra.
The 40 days of the summer struggle correspond to the 30 dog days of the Roman-European tradition. For similar mythological-astrological entanglements like that between Sirius / Tishtrya and water / fertility see Sirius in Sumer and Mesopotamia as well as Sirius in Egypt . An influence of these cultures on the Zoroastrian-Iranian culture (especially in astrological-calendar matters) is generally assumed.
In today's Islamic Iran, the old tradition of hard summer and winter days exists in a greatly changed form and with a Sufi interpretation. Only the name of these periods - chelleh “forty” - still points to the pre-Islamic tradition, but neither of the two periods is perceived as 40 days. A touch of Zoroastrian tradition was revived in the Iranian calendar of 1925 , whose month "Tir" (Avestisch Tishtrya ) is again in July-August, thus again roughly corresponding to the dog days. The winter period was also linked to the solar events, beginning on the night of the winter solstice (i.e. understood as such by the general Iranian population), and ushered in a festival that continues to be known as Shab-e Chelleh “Night of Forty (Days)” is.