From the Diwali Wikipedia page:
Diwali (or Deepavali, the "festival of lights") is an ancient Hindu festival celebrated in autumn (northern hemisphere) or spring (southern hemisphere) every year. Arguably the most important festival in Hinduism, it is an official holiday in Fiji, Guyana, India, Pakistan, Malaysia, Mauritius, Myanmar, Nepal, Singapore, Sri Lanka, Suriname and Trinidad and Tobago. The festival spiritually signifies the victory of light over darkness or good over evil, knowledge over ignorance, and hope over despair. Its celebration includes millions of lights shining on housetops, outside doors and windows, around temples and other buildings in the communities and countries where it is observed. The festival preparations and rituals typically extend over a five-day period, but the main festival night of Diwali coincides with the darkest, new moon night of the Hindu Lunisolar month Kartika. In the Gregorian calendar, Diwali night falls between mid-October and mid-November.
Before Diwali night, people clean, renovate, and decorate their homes and offices. On Diwali night, Hindus dress up in new clothes or their best outfit, light up diyas (lamps and candles) inside and outside their home, participate in family puja (prayers) typically to Lakshmi – the goddess of fertility and prosperity. After puja, fireworks follow, then a family feast including mithai (sweets), and an exchange of gifts between family members and close friends. Deepavali also marks a major shopping period in nations where it is celebrated.
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 and 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.
On the same night that Hindus celebrate Diwali, Jains celebrate a festival of lights to mark the attainment of moksha by Mahavira, Sikhs celebrate Bandi Chhor Divas and some Newar Buddhists also celebrate Diwali remembering Ashoka's conversion to Buddhism.
The word Diwali means rows of lighted oil lamps.