The Ramayana. One of the many sacred stories associated with Diwali.
It is supposed to be the night of celebration in all of Ayodhya, Prince Rama's kingdom, when he returns back to his kingdom with his wife, Sita, and his younger brother Lakshman after defeating the evil Ravana, the demon ruler of Lanka.
One of two sacred Hindu epic mythologies of all time. The other one being, of course, the Mahabharata.