The Full Moon Day or 15th Purnima day of the Shukla paksha (waxing moon) of the Kartika month is considered very auspicious. The full Moon is usually in Taurus and therefore exalted. At times this festival can take place with the full Moon in Aries- but it represents a strong and auspicious Moon. Usually this is the full Moon in the month of November. It has many names including the Diwali of the Gods and Tripuri Purnima. This day marks the victory of Lord Shiva over the demons Tripuri. The Kartik Purnima festival also coincides with the Jain light festival and Guru Nanak's Birthday.
Tripuri Purnima is one of the most propitious day for worshipping of Shiva, next only to the Maha Shivaratri. According to Mahabharata there were three sons born to the demon Taraka. Kartikeya (Mars)killed Taraka. Taraka?s sons were called Taraksha, Kamalaksha and Vidyunmali. Demons in Vedic philosophy are very intelligent and strong but they tend to use their knowledge and strength towards the wrong means. These demon sons of Taraka had performed severe penance towards Brahma and won the gift of immense power. The boon that they received, granted them that they would live for a thousand years in three invincible, moving cities known as Tripuri and that they could only be destroyed by one arrow that would merge the three cities into one, and set them to fire. Tri means three and Puri means cities. Tripuri were resplendent, made up of gold, silver and iron respectively. They were built to rival Indra?s grand city. As such powerful demons, the Tripura spent all their time and energy to harass the gods. The power was such that it was difficult to control their destructive tendencies. Their cities never remained in one place long enough therefore it was difficult to destroy them together.
At the end of a thousand years of their negative rule, the gods went to Rudra (Shiva) and asked him to destroy the demon. Rudra had to seek the help of Brahma and the other gods in this task. He danced Tandava, his dance of destruction. He rocked the three cities with his dance (The Tandava of Shiva is said to be like an earthquake); his arrow pierce through the three demons at one time and the fire from his third eye burnt the three cities.
In commemoration of burning and destroying the demons and their cities, this event, which occurred on Kartik Purnima day, is observed annually. While Diwali is the Rama?s (God in a human form) triumph over the demon, Dev Diwali celebrates the Global fight against evil forces that disturb the harmony of the universe and saving of the whole universe from the destructive intention of those with negative gaendas. While Brahma can create, how people use their creation is up to them. It is Shiva who can save the world when the creation goes wrong. This form of Shiva is known as Tripurantaka for killing the three demon princes and destroying their evil cities.
It is believed that annually on the day of Kartik purnima, the gods descend on earth and reside in the sacred rivers. Ganges and other sacred rivers have elaborate ceremonies on this day. Devotees who bathe in the sacred rivers on these days can also get rid of their negative poisons and receive the blessings from all the Gods of the India.
To celebrate this day we can also cleanse our inner selves. To bathe our mind in the spiritual waters even if we do not live near sacred rivers. Allow the spiritual light to burn away all the questionable traits within. It is not the show of cleaning the outer selves but to rid of our mind and life of its wrong qualities like fear, greed, envy, anger, arrogance and wrong desires and replacing those with forces of love, compassion, generosity, kindness, and light.
— Komilla Sutton