Though Kartik Poornima is not treated as a full-fledged festival, there are enough mythological stories which suggest that this is a very important day in Hindu religion.
Both moon days of the month Kartik, (full moon and no moon) are auspicious. Many will not comprehend Kartik Amavasya, as it is better known as Diwali. Kartik Poornima has even more importance as it is called Dev Diwali ( the diwali of gods). This is the day when gods celebrate the victory of good over evil.

This festival underlines the importance of Lord Shiva. This festival is second only to Mahashivratri. Shiva relieved the earth by killing Tripurasuras.
Tripasura are the three sons of  Tarakasura – Vidyunmali, Tarakaksha and Viryavana. Because of a boon from Lord Brahma, they were virtually immortal. They were living comfortably in the city of Tripura. Gods pleaded Shiva to destroy them which Shiva denied. Then they pleaded Vishnu and he found a solution to their problem. Vishnu created a person to propagate an alternate form of religion. This religion was slowly accepted by the three sons of Tarakasura. They stopped following the teachings of Veda and worshipping Shiva. Lord Shiva then destroyed the city of Tripura with Pashupata astra. The chariot was created by Vishwakarma.

For this reason, Lord Shiva is also called Tripurari.

Kartik Poornina is also associated with following important occasions:
1. Birth of Kartikeya, son of Lord Shiva.
2. Birth of Matasya (Matsya) avatar, an incarnation of Vishnu.
3. Birth of Vrinda, the personifield form of Tulsi.

The festival is more significant when this is in the Krittika nakshatra. It is then called Maha Kartik. If the nakshatra is Bharani, the results are stated to be special. In Rohini nakshatra, then the fruitful results are even much more.
A ritual bath at a tirtha (a sacred water body like a lake or river) at a pilgrimage centre is prescribed on Kartik Poornima. This holy bath is known as “Kartik snana”. An holy bath at Pushkar or in the Ganges river, especially at Varanasi is deemed as most auspicious. Kartik Poornima is the most popular day for bathing in the Ganges at Varanasi.

This auspicious day is also celebrated in others religions – Sikh (Guru Nanak Jayanti) and Jain (Shri Shantrunjay Teerth Yatra).