After Deepavali, Hindus or Indians celebrate Dev Deepavali.
What is the reason for celebrating Dev Deepavali?
Does this festival or tradition find mention in the Puranas?
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Dev Diwali is celebrated (in Varanasi) on the day of Kartika Poornima (कार्तिक पूर्णिमा) which is also known as Tripura Purnima (त्रिपुर पूर्णिमा) according to Hindu Panchang.
The mythological story (पौराणिक कथा) related to this is about slaughter of demon/devil "Tripura" (त्रिपुर) by lord Shiva with the help of Vishnu & Brahma. And Devas celebrated this day like Diwali.
You can find Scriptural Reference for this story in Srimad Bhagavat MahaPurana (7.10.52 to 7.10.71).
SB 7.10.56 — Thereafter, when the demons had begun to destroy the higher planetary systems, the rulers of those planets went to Lord Śiva, fully surrendered unto him and said: Dear Lord, we demigods living in the three worlds are about to be vanquished. We are your followers. Kindly save us.
SB 7.10.67 — My dear King Yudhiṣṭhira, the most powerful Lord Śiva joined the arrows to his bow, and at noon he set fire to all three residences of the demons and thus destroyed them.
SB 7.10.68 — Seated in their airplanes in the sky, the inhabitants of the higher planetary systems beat many kettledrums. The demigods, saints, Pitās, Siddhas and various great personalities showered flowers on the head of Lord Śiva, wishing him all victory, and the Apsarās began to chant and dance with great pleasure.
SB 7.10.69 — O King Yudhiṣṭhira, thus Lord Śiva is known as Tripurāri, the annihilator of the three dwellings of the demons, because he burnt these dwellings to ashes. Being worshiped by the demigods, headed by Lord Brahmā, Lord Śiva returned to his own abode.
This may be the belief/mythology related to the celebration of Deva Diwali.