Full moon day (Purnima) in the month of Kartik in a traditional Hindu calendar is called Kartik pournima , also known as Tripurari pournima ,Dev Diwali. Dev Deepavali is the Diwali of the Devas or Gods held on the Kartik pournima as It is believed that on the day of Dev Deepavali, the Gods descend on Earth. Tulsi Vivah is also carried out this day.
An important ritual on the day is the lighting of Diyas (lamps) after the sighting of the moon. In some parts "Tripurasur Dahan" (Burning) is also carried out.
Kartik Snan (Holy bathing) and Deepdaan are main rituals performed during Dev Diwali. At the time of sunrise devotees do Kartik Snan and in the evenings they perform Deepotsav.
During this Dev Diwali, a also a cultural festival called ‘Ganga Mahotsav is celebrated in Varanasi. Prabodhini Ekadashi is the important day in these celebrations. Thousands of people visit Dasaswamedh Ghat to see liting of thousands of Diyas (earthen lamps) this is a tribute to river Ganga by people of Varanasi.
Stories associeated with Tripurari Pournima or Kartik pournima -
It's said that Lord Vishnu returned to Vaikunta, his abode, on this day after the Vamana incarnation and visiting King Bali. Kartik Poornima is closely associated with Prabodhini ekadashi , which marks the end of Chaturmas , a four-month period when Vishnu is believed to sleep. Prabodhini Ekadashi signifies the awakening of the god.
Kartik Purnima is also celebrated as the birth of Lord Vishnu's first incarnation, the Matsya (fish) avatar. It is also the birthday of Vrinda (Tulsi plant) and of Lord Kartikeya, the son of Lord Shiva.
Another story is that Lord Shiva killed three demons, who spreading Adharma and killing innocent people, and destroyed the three cities built by the demons on Dev Diwali day. Therefore the day is also known as Tripurari Purnima.
We find this story in ShreemadBhagvatam - (7 skanda chapter -10 shloka 53-70)
अथानुगृह्य भगवन्म भैशेष्ट इति सूरान्विभु: ।
शरं धनुषि सन्धाय पुरेष्वस्त्रं व्यमुश्र्चत ।।57।।
Meaning - On hearing their request , Lord Shankara told the Gods "Do not fear" .Then he shot arow towards these 3 cities.
एवं दध्वा पुरस्तिस्त्रो भगवान्पुरहा नृप ।
ब्रह्मादिभि: स्तुयमान: स्वधाम प्रत्यपद्यत ।।70।।
Meaning - O Yudhishthira , thus by burning those 3 cities , Bhagvan shankar became "Tripurari" & went to his abode with hearing
the praising by Brahmadeva etc.