Diwali is the Indian "festival of lights" where light overcoming darkness is celebrated. Symbolically-speaking, the light and darkness may represent good overcoming evil or knowledge overcoming ignorance. Although recognized as a national holiday in India and other South Asian countries, it is primarily celebrated by followers of Hindu and Jain faiths.
One of the most common ways of celebrating this holiday is by illuminating the home - through the use of candles, lanterns, or diyas (a specific type of oil lamp) - and gathering with the family. These gatherings traditionally include a feast comprised of special dishes and sweets. Prayers may be made to ancestors if the family or individual celebrants have an ancestral practice. There are certain deities connected to this holiday - Lakshmi and Kali among them.
Because this is a holiday determined by the lunar cycle, the official date changes annually between the period of mid-October and mid-November. This year, Diwali is on Sunday, November 12th, but the celebration itself can carry over into several days across that timeframe.