This Festival is celebrated on the 13th day of the month of Tir, (the 4th month of the Persian calendar) which corresponds to the 2nd or 3rd July in the Gregorian calendar. This Zoroastrian Rain Festival of Tir Jashan, dedicated to the star deities of Tir and Teshar. It was one of the three most widely celebrated feasts (along with Mihragan and Noruz) of ancient Iran, and is  mentioned in the Talmud. 

Tir,  is the Yazad which presides over Sirius, the brightest star in the sky, and of rain and thus is invoked to enhance the harvest. It is customary to give bracelets made of the seven colours of the rainbow. At the end of the festival believers go to a high place and throw them into the wind. 

Children have lots of fun splashing and ducking each other in lots of water.

Further Information

For further information see https://www.tehrantimes.com/news/437662/Tirgan-a-feast-in-honor-of-rain