On this most auspicious day, Sikhs as well as Hindus offer prayers to the sacred river Ganga and take a holy dip also known as Snaan.
The significance in Sanatan Hindu Dharm goes back to over millions of years when it is believed that Lord Shiva defeated the demons or Asuric forces of Tripura Asura. Shiva in His form as Tripurantaka (Killer of Tripurasura) killed Tripurasura on this day. Tripurasura had conquered the whole world and defeated the Devateys and also created three cities in space, together called "Tripura". The killing of the demon(s) and destruction of his/their cities with a single arrow - by Shiva overjoyed the Devateys and they pronounced the day as a festival of illuminations. This day is also called "Dev-Diwali" - the Diwali of the gods. Diwali is the Hindu festival of lights.