Pournima

 Tripurasura (Sanskrit: त्रिपुरासुर) is a trio of asura brothers named Tarakaksha, Vidyunmāli and Kamalaksha, who were the sons of the asura Tarakasura. These three began to perform severe tapasya. They were then granted boons by Brahma to have three forts: gold, silver, and iron, which angered the Devas. Vishnu then made a new religion to make them evil, and the objective of killing the asuras was taken upon by Lord Shiva, which took three days on the battlefield, finally killing Tripurasura and destroying the three cities. This occurred on the full moon day in the 

Kartik month and therefore the day is celebrated as Tripurari Pournima.

https://www.google.com/search?q=Tripurari+Purnima