This year Diwali is celebrated on Wednesday November 11, 2015. So exactly what is Diwali? Diwali, derived from the Sanskrit fusion word Dīpāvali, is a celebration of millions of lights that dates back to ancient India. It is one of the largest and brightest festivals in India. Spiritually, this festival signifies the victory of good over evil. Preparations and rituals typically extend over a 5-day period. This festival coincides with the darkest, new moon night of the Hindu Lunisolar month Kartika. Before the night of Diwali, people clean, renovate and decorate their homes and offices. Diyas (lamps and candles) are lit inside and outside of homes.
According to Wikipedia,
On the same night that Hindus celebrate Diwali, Jains celebrate a festival of lights to mark the attainment of moksha by Mahavira, and Sikhs celebrate Bandi Chhor Divas. Diwali is an official holiday in Nepal, India, Sri Lanka, Myanmar, Mauritius, Guyana, Trinidad and Tobago, Suriname, Malaysia, Singapore, Fiji, and the Australian external territory of Christmas Island.
The name of festive days as well as the rituals of Diwali vary significantly among Hindus, based on the region of India. In many parts of India, the festivities start with Dhanteras(in Northern & Western part of India), followed by Naraka Chaturdasi on second day, Deepavali on the third day, Diwali Padva dedicated to wife–husband relationship on the fourth day, and festivities end with Bhau-beej dedicated to sister–brother bond on the fifth day. Dhanteras usually falls eighteen days after Dussehra.