Diwali comes over trumpt of Ravna. Its directly related to Ram & Ravna. Ram is Great social man. On The other hand Ravna also great king of Lanka. But he consider no one king in the universe. So Ram & Ravna fight each other lastly Ram trumpt over Ravna. when Ran come to Ayodhya afetr wining the war. Ayodhya washi welcomed him to burnt light . Thats culture follow India in many way inn different state. For more you can read or contact any mahashi or Ramayana.