Known in India as the "Festival of Lights," the Hindu holiday Diwali is a cause for celebrations on a scale that should make Hanukkah blanche. The celebration is intended to remind observant Hindus of goods invariable triumph of good over evil and the concept of "Atman," which is quite similar to the Christian idea of the soul. The visual metaphor used to express the holiday's buoyant optimism is the lamp, so Indian cities twinkle throughout the celebration.
Though the holiday is celebrated well beyond India's border, not only in Bangladesh but also in places that have large Hindu populations like Guyana and the U.S., the celebration is probably best enjoyed in person in India's less commercial cities like Allahabad, where locals pour an amazing amount of effort into making the celebration memorable, dressing as gods and stringing up lanterns. Jaipur, in the Rajasthani desert, puts on a show with its lights as does Amritsar, where the Golden Temple is illuminated.
All that said, if travelers aren't in India well before the holiday begins, getting anywhere can be extremely difficult as families tend to use the holiday to reconnect. This chaos makes Thanksgiving travel look like a picnic.
It is perhaps best then to get a sense of the celebration with some photos.