Chak Chak is the most important of the Iranian pilgrimage shrines, and the account attached to it is unusually clearly marked as story rather than history.
The place
A shrine set into a cliff face in the desert north of Yazd, reached by a long flight of steps up the rock and entered through bronze doors.
Inside is a grotto where water drips continuously from the rock, which is what the name describes: chak chak, the sound of the drops. An ancient tree grows at the entrance.
Water emerging perpetually from bare rock in one of the driest landscapes on earth requires no elaboration to be remarkable, and the shrine is built around that fact.
Its other name is Pir-e Sabz, the green shrine.
The account
In 640, as the Arab armies completed the conquest of the Sasanian empire, Nikbanou, a daughter of the last king, Yazdegerd III, fled eastward and was pursued to this cliff.
Cornered, she prayed to Ahura Mazda for shelter, and the mountain opened and took her in.
The water is her tears, weeping for the empire. The tree at the entrance grew from her staff.
How it is held
This is legend, and the community presents it as such. It is not offered as history and Zoroastrians do not generally argue for its literal truth.
Its interest is in what it does rather than whether it happened.
The story fixes the moment of the conquest, the end of Zoroastrianism as the religion of a state, and the beginning of fourteen centuries as a minority, to a particular rock, and returns the community to that rock once a year to weep with it.
A religion that lost an empire needs somewhere to put that loss. Chak Chak is where it is put, and the perpetual dripping makes the grief continuous rather than commemorative.
The pilgrimage
The main pilgrimage runs from 14 to 18 June. Thousands come, from Iran and from the Indian Parsi community.
By custom pilgrims stop their vehicles at the foot and walk the final approach, and there are strictures about conduct in the grotto.
Context
Zoroastrians in Iran are a small and legally constrained minority, recognised in the constitution with a reserved parliamentary seat but subject to real restrictions.
For that community, an annual gathering of thousands at an ancestral shrine is not principally a religious observance in the narrow sense. It is one of the few occasions on which the community assembles, in numbers, visibly, at a place of its own, which is a reasonable part of why the pirs matter as much as they do.