Tirmā.Sizz∂.Šu is a festival remnant of Tirgān, celebrated since Indo-Iranian period. It is still celebrated in some parts of northern Iran, especially by the alpine natives of Māzandarān, on 13th of Abān (Nov). Classical historians, relying on Biruni's tradition in Athār alBāqiye (Chronology) mentioned two relations: 1) archery of Arash the archer; 2) the struggle with drought and rain-quest. Later researchers have also presumed the origin of this Aryan festival in the myth of Arash, cultivation and rain-quest simultaneous with the rise of Tistrya Star (Sirius).The new hypothesis suggested in this paper, denies any original relationship between the festival with the myth of Arash, but it is the ritual aspect of the myth of Tištrya, the star and the Deity of rain. According to this Indo-Iranian myth, the Avestan Tištrya, the counterpart of the Vedic Tišya, fights with the demon of drought, overcomes him, frees and spreads the water throughout the world. The motif of fertility and rain-quest in summer was so significant that the myth of Tišya / Tištrya found a ritual function in the eyes of Indo Iranians, and consequently it is more significant than the myth of Arash. As for, how this summer festival comes forth to Autumn in the northern parts of Iran, is by itself another subject of this paper.