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Tihar derives from the Sanskrit words Deepa and avail which literally mean "lights in rows". It is related to several Hindu myths. These myths all tell the story of justice over injustice and light over darkness.
One of the myths tells that Hindu god Krishna killed the Narakasura, a mythical Asura king, who intended to destroy the world. In northern India, Hindus worship the god Krishna, who lives in the sacred Mount Gfadhana and is regarded as the eighth avatar of Vishnu, one of the main Hindu gods. This myth has deep religious significance for the followers of Vishnu.
While the most commonly known story for Indians in the Southern region is to recall the return of God Rama after 14 years away from his hometown, during which he defeated the demon king Ravana and finally returned to Ayodhya, the oldest city in India. To celebrate this, the people lighted thousands of pottery lamps.
As the Brahmin described:  "this city held a grand and joyous celebration for the return of the king. The shining lights symbolize how brilliant the heroic image of the king. And the name of the festival highlights the joy of people."
more info:tihar in 2016