Tripurasur had three cities, one of iron, one of silver, one of gold, and they moved — they could only be destroyed when all three stood in a line, and that happened once in a thousand years.
Shiv ji waited for it and let one arrow go. The night that followed is Tripuri Purnima, and it is why the whole sky over the river is lit on this night — Dev Diwali, the gods' own Diwali, fifteen days after ours.
The same full moon is Guru Nanak's birth, and in Jain houses it is the day the Siddhachakra is kept. Three reasons, one river, and the lights go on the water for all of them.
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