Dīpavăli (also transliterated Deepavali; Sanskrit: row of lights) or Diwăli (contracted spelling) is the Hindu festival of lights, held on the final day of the Hindu calendar (compare New Year's Eve). The following day, marking the beginning of a new year, is called Annakut.
Dipavali falls in the Gregorian month of October or November, and always on a new moon day. Since the precise moment of the new moon falls on different Gregorian dates depending on geographical location, the date of Dipavali also depends on one's location.
It is celebrated by Hindus all over the world, every year. On the day of Dipavali old and young, rich and poor wear new dresses and share sweets. They also burn crackers. The traditional business community starts their financial new year on Dipavali and new account books are opened on this day.
There are two mythological legends associated with Dipavali. The first Dipavaliwas held to celebrate the return of the Rama, King of Ayodhya, his wife Sita and brother Lakshmana to Koshala after a war in which he killed the demon Ravana. It was getting dark, so people along the way lit oil lamps to light their way. Second, it commemorates the killing of Narakasura, who was also an evil demon. So Dipavali is a festival symbolising the destruction of evil forces.
There are various legends relating to Dipavali as also different ways of celebrating in different parts of India. Dipavali is celebrated over 5 days in most of north India as:
In South India, naraka chaturdashii is the main day of celebration with lot of fire crackers at dawn while in North India the main celebration is on Amavasya evening with Lakshmi Puja followed by lighting of oil lamps in and around the house and bursting of crackers.
In England, the days are Dhanteras, Narak Chatrudashi, Lakshmu-Puja, the most important day, Padwa or Varshapratipanda and Bhaiya Dooj or the Teeka Ceremony  (http://www.leicester.gov.uk/departments/page.asp?pgid=6490).
The time is also significant to Sikhs. During the festival time in 1620 the 6th Guru, Hargobind Singh gained the release of 52 Hindu princes who had been falsely imprisoned in Gwallior Fort by the rulers of the area, the Mughals. The Golden Temple was lit with many lights to welcome the release of Guru Hargobind and Sikhs have continued the celebration.
Criticism related to firecrackers
In recent years there has been some criticism about the celebration of Deepavali in India. The most common reason is the noise pollution caused by firecrackers, which particularly affects infants, pets and older people whose sleep can also be disrupted by firecracker noise that continues late into the night.
The noise from crackers has a more deleterious effect on animals since they have a more sensitive sense of hearing than humans. Pets like dogs and cats as well as stray cattle in cities spend the Deepavali days in a state of confusion. Voluntary and non-profit organizations like People for Animals educate the public about these issues. Additionally, Smog is extremely common on the morning after Deepavali, and may be harmful to inhale and causes difficulty for drivers through reduced visibility.
Deepavali can often be treated by some as an opportunity to show off their status or purchasing power. This competitive approach encourages the use of ever larger and noisier fireworks.
Recently there have been several governmental and legal efforts to combat the menace. The Supreme Court of India, observing that the "right to peaceful sleep is a fundamental right of the citizens", has banned crackers between the hours of 10 pm and 6 am during the Dasara and Diwali festivals. While strict enforcement of this ban is not yet in place, the effect has nevertheless been very positive.
The Central Pollution Control Board has banned fire-crackers with a decibel level of more than 125 at a distance of 4 meters from the bursting point. There have also been state-level efforts to ban the very loud "1000-walas" and "hydrogen bombs" fireworks.
There have been efforts by some non-governmental organisations to educate school children about the ills of firecrackers, because it is understood that children are the ones who influence their parents to purchase them and are the ones who eventually light them. The cumulative effect of these actions has been a noticeable reduction of noise during Deepavali.
There is also the issue of child labor in the fireworks industry whose main centre in India is Sivakasi in Tamil Nadu. Children as young as three or four (the average is 10-14) work in harsh conditions, and about a third of them are in debt bondage. There is some public awareness of this problem, but the longstanding issue of child labor in India is larger than the context of Deepavali alone.
The festival is interesting enough around the world that the search for diwali greetings (http://www.google.com/search?q=diwali+greetings) was the 2nd of the top 10 gaining Google queries for the week ending October 27, 2003  (http://www.google.com/press/zeitgeist/weeks-oct03.html).