The festival of light, celebrating Rama’s (the light) victory over the demon Ravana (the dark).
The death of the demon Naraka is also associated.
http://hindumythologyforgennext.blogspot.com/2012/07/death-of-narakasura-origins-of-diwali.html
Another Diwali story
http://utkarshspeak.blogspot.com/2012/11/diwali-and-gambling.html