The word Diwali is a contraction of Deepavali, originating from the Sanskrit word Dipavali which can be translated to Row of Lights. Hence the Diwali Festival is also called the Festival of Lights. Diwali is the name for the festival in North-India. In South-India the festival is called Deepavali.
Diwali celebrates to victory of the Good over the Evil and Light over Darkness. Is has a major religious significance for Hindus, Sikhs and Jains alike - not only in India, but also for Indians living abroad. In the western (gregorian) calendar, Deepavali falls on a day in October or November every year - just after the monsoon season in India.
The exact date varies and is being calculated based on the Hindu Luni-Solar calendar (according to the positions of the Sun and the Moon). The day of Diwali falls on Ashvina Amavasya (the lunar day of new moon) on 15 Ashvin (Hindu month). This date also marks the beginning of the Hindu New Year, and many businesses in India starting a new accounting year on the Deepavalis holiday.