Complex sentence: A popular feast is reported to have been held in the city of Damavand on 7 Šawwāl 1230/31 August 1815, during which the people celebrated the anniversary of Zahhaks death.
Split into simpler sentences: A popular feast is reported to have been held in the city of Damavand on 7 Šawwāl 1230, or in Gregorian calendar, 31 August 1815. During the alleged feast the people celebrated the anniversary of Zahhaks death.