Poornima in Shukla Paksha of Kartik month is called Kartik Purnima. On this day, Lord Shiva killed the demon, Tripurasura, therefore this Poornima is also known as Tripuri Poornima. If Krittika Nakshatra falls on this day, it is called Mahakartiki. If it is Bharani Nakshatra, then a person is satisfied with favorable results. And if it is Rohini Nakshatra on this day, the significance of Kartik Purnima increases tenfold.
It is assumed that Lord Vishnu took Matsya Avatar on the eve of Kartik Poornima. Lighting lamps and donate things after Ganga Snan (bathing in Ganga River) on this day, give results equivalent to the results of 10 Yagyas. Kartik Purnima has also been called a very favorable festival by the Gods (Brahma, Vishnu, Shiva, Angira and Aaditya).
Ganga Snan, lighting lamps (Deep Daan), Hom (Aahuti), Yagya and worshipping Gods during Kartik Purnima hold great importance. The rituals that are performing on this day are as follows:
Purnima in the month of Kartik is considering among the most pious Poornima in a year. Gift and charity deeds perform on this day offer honest results. On Kartik Purnima, if Moon is in Krittika Nakshatra and Sun in Vishaka Nakshatra, it creates Padmak Yoga which is very rare. And if it is Moon and Jupiter in Krittika Nakshatra on this day, the Purnima is called Maha Purnima. By celebrating Tripura Utsava and lighting lamps (Deep Daan) in the evening of Kartik Purnima, the pains and suffering of previous life end.
In ancient time, there was a demon named Tripura. He performs very difficult penance (Ghor Tapasya) in Prayagraj for one lakh years. Seeing his devotion, all living creatures and deities (Devta) got scared. So, Devtas sent nymphs (Apsara) to rupture his Tapasya, but it was useless. Pleased with the tenacity (Tapasya) of Tripurasura, Lord Brahma appears before him and asked him to make a wish.
Tripurasura wished, I should neither be killed by a deity nor a human. Brahma ji granted him the wish, after which, the demon got fearless and began to torment everyone. Not only had that, but he even gone to Kailash to conquer it. There he had a battle with God Shiva, and in the end, the God killed him with the help from God Brahma and God Vishnu.