When the sun sets, Zoroastrian devotion turns to the fravashis. These are among the religion's most distinctive conceptions: pre-existent spirits of the righteous, at once guardian angels, ancestral souls and the eternal prototypes of persons, who chose before time to enter the material world and fight for the good creation.
The Aiwisruthrem Gah, spanning sunset to midnight, is their watch, and its Avestan prayer invokes them together with the beings of the protected evening world: the text remembers the flocks brought home at dusk, and the tradition associates the watch too with Verethraghna, the yazata of victory, whose strength guards the community when the light does not.
The theological logic is protective. Night in Zoroastrian cosmology inclines toward the powers of disorder, and the evening watch answers by calling up the largest army the religion knows, the countless fravashis of all the righteous who have ever lived, whose festival at year's end, the Frawardigan days, extends the same devotion into ten days of household remembrance.
In practice the watch has always been domestic: the hearth fire fed and saluted, the lamps lit with prayer, the departed of the family remembered by name, the kusti retied and the gah prayer said before sleep approaches. Where Havan belongs to the fire temple and Uzerin to the waterside, Aiwisruthrem belongs to the house, and it made every Zoroastrian home, at nightfall, a small fortress of invoked guardianship.