Dev Deepawali or Diwali is a festival celebrated on Purnima of Karthik Maas. On this day people worship Lord Shiva and light Diyas at night on their doorsteps. In Varanasi, special Puja is held for this day and at night, Ghats of mother Ganges are decorated beautifully with flowers and diyas.
According to Hindu mythology, on this day Lord Shiva rescued the Devtas from the prison of Tripurasura. Tripurasura is the name given to the three demons Tarakaksha, Viddyunmali and Kamalaksha. After a hard penance three of them received bliss of being killed only by Lord Shiva and no one else could defeat them and also three of them got one palace each i.e. one of gold, one of silver and one of iron. These three palaces were always in a constant state of motion and never came in a line.
These palaces were designed in such a manner that to destroy them, all three should be attacked at the same time. After this, the trio also worshipped Lord Shiva and Lord Shiva told them that he would not harm them until they harm religion. After this they started what all other Asuras do after getting power. But because they had protection of Brahma and Shiva, no one could say a word against them. They made a strategy to kidnap all the Devtas and prison them in their palaces.
After this Shiva decided to stop these demons but for this he needed a special technique of warfare. He called Vishwakarma, the chief architect of Devtas. Vishwakarma designed a chariot, a bow and an arrow. These weapons were not the usual weapons, chariot was all of gold, the string of bow was the Nagaraj Vasuki himself and the arrow was Pushpastra, the divine weapon.
Shiva aimed the arrow in such a way that all the three palaces were destroyed at the same time and all the Devtas were free. After this Shiva killed the three demons and all of them returned. They returned to Swarga on Purnima of Karthik Maas. And whole Swarga celebrated this day as Deepawali. On this day, Devtas thank Lord Shiva and enlightened the earthen lamps at Varanasi, which is considered as Lord Shiva’s house.
From that time it is a custom to light Diyas at Ghats of Ganges. For well being of family and to have blessing of Devtas one should light earthen lamps before Shivlinga in any temple around your home. It is believed that Lord Shiva provides protection to those who follow this custom.