The full moon day of the Kartik month is considered extremely sacred. Especially in North India, taking a bath in the Ganges on this day holds great significance. Although people in rural areas go for a bath in the Ganges or nearby rivers on every full moon and Ekadashi day, on this day, the Ganges bath is considered exceptionally pure.
Let's now look at its scriptural aspect as mentioned by Astrologer Anshul Pandey.
According to the Narad Purana (First part, Chapter number 124), to achieve victory over all enemies, one should have the darshan of Kartikeya on the full moon day of Kartik month. On the same day, during the Pradosh time, one should perform the 'Tripurotsav,' a joyful offering of lamps to bring happiness to all living beings. On this day, by seeing the lamp, one attains liberation. During the moonrise, worship should be done with flowers, incense, lamps, abundant offerings, excellent food, fruits, vegetables, etc., and through homa (ritual fire).
Thus, by worshiping the deities and offering lamps outside the house, a beautiful square pit should be dug near the lamps. It should be filled with cow's milk and a beautiful golden fish should be placed in it. The fish's eyes should be made of pearls. Then, chanting the mantra 'Mahamatyaya Namah,' the fish should be worshipped with fragrances, etc., and donated to a Brahmin. This is the method of offering the Ksheera Sagar (milk ocean) gift. Through this donation, a person enjoys bliss near Lord Vishnu. By observing the 'Vrishotsav Vrat' and 'Naktavrat' on this full moon day, one attains Rudra-loka.
According to the Vratutsav Chandrika (Chapter number 31), from ancient times, this day is known as Katki or Kartiki. On this day, Lord Vishnu's Matsya Avatar also took place. Behind the sanctity, there is a story that on this day, Lord Shiva killed an Asura named Tripurasura. Tripura had spread terror and chaos everywhere. Through severe penance, he obtained a boon that he could not be killed by any man or woman, deity, or demon. Only someone without parents could kill him. Although Tripura wanted immortality, Brahma said that he couldn't grant such a boon. So, Tripura had to be content with that. However, his unrest increased. He made the demons his doorkeepers, causing terror everywhere.
One day, Narad reached Tripura. His visit always has a purpose. After inquiring about the well-being, Tripura asked Narad if there was anyone more powerful than him. Upon learning that he was the most powerful, Tripura became arrogant and destructive. On the other hand, Narad went to the gods and told them why they were tolerating this terror. When the Apsaras (celestial nymphs) sent by the gods to trap Tripura in a net failed, they went to Brahma. Brahma explained that since he himself was the giver of the boon, he couldn't do anything. The gods then went to Vishnu. Vishnu explained that in reality, everything applicable to Lord Shiva. So, all the gods went to please Lord Shiva, and he agreed.
Lord Shiva, armed with a bow and arrow, started killing the demons. After that, Lord Shiva killed all the demon Asuras and Tripura himself, restoring the gods to their immortal abode. I have detailed this story in my book ('Authentic Concept of Shiva'). Therefore, today is considered a sacred day. On this day, after bathing in the Ganges, performing charitable acts eliminates all sins and troubles.
On Kartik Purnima, we also celebrate Guru Parv as Sikh brothers and sisters. On this day, Guru Nanak was also born. People also call this day 'Dev Diwali.' It is the last day of Diwali and the Kartik month."