From a Wikipedia page :

"Tirgan is an ancient Iranian tradition which is still celebrated in various regions of Iran, including Mazenderan, Khorasan, and Arak.[1][2][3] It is widely attested by historians such as Gardezi, Biruni, and Masudi, as well as European travelers during the Safavid era.

The celebration is dedicated to Tishtrya, an archangel who appeared in the sky to generate thunder and lightning for much needed rain.

Legend says that Arash the Archer was a man chosen to settle a land dispute between the leaders of the lands Iran and Turan. Arash was to loose his arrow, on the 13th day of Tir, and where the arrow landed, would lie the border between the two kingdoms. Turan had suffered from the lack of rain, and Iran rejoiced at the settlement of the borders, then rain poured onto the two countries and there was peace between them.

It is stated in Biruni's chronology that "by the order of God, the wind bore the arrow away from the mountains of Ruyan and brought the utmost frontier of Khorasan between Fergana and Tapuria."[4] Gardizi has given a similar description, although he notes that "the arrow of Arash fell in the area between Fargana and Bactria."

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