Observed after fourteen days of Diwali, it marks a tribute to river Ganga. It is a popular belief that on this night (full moon); Gods descends on earth to bathe in the Ganges. Another legend speaks of the return of Lord Vishnu to his abode after the Vamana incarnation. Thus it has become an old tradition in the entire city to lit lamps to welcome Gods.
There are regular trains running to Banaras from New Delhi. You can travel overnight to reach here. Best and cheapest accommodations are available at Asi Ghat. There are a lot of roof top Guest Houses from where you can see see the Ganga flowing below. It is an amazing sight in the morning. If you wish to visit Banaras or Varanasi during Dev Diwali then you should make sure that all your bookings are done in in advance. You must take out time for boating as it is the spectacle from middle of the river.