tamaso ma jyotirgamaya
Lead me from darkness to light

But of course, one can be a bit more comprehensive:

asato ma sadgamaya
tamaso ma jyotirgamaya
mrtyorma amrtam gamaya

Lead me from the untruth to the truth.
Lead me from darkness to light.
Lead me from death to immortality.

(Brhadaranyaka Upanishad — I.iii.28)

With so many Goddesses and Gods in everyday life, there is a festival just about every week in all parts of India. However, Diwali is celebrated with fervor and gaiety by young and old, rich and poor, throughout the country to dispel darkness, light up their lives and bring in the new year. Diwali is also called the Festival of Lights.

The festival symbolizes unity in diversity as every state celebrates it in its own special way.

The most common celebration is about Rama and Sita: Lord Rama, a great warrior King, was exiled for fourteen years by his father Dashratha, the King of Ayodhya, along with Rama's wife Sita and younger brother Lakshman, on Dashratha’s wife's insistence. After adventorous periods in the forests across India during which time Rama fought and killed the mighty demon Ravana, Lord Rama returned to his Kingdom Ayodhya where the people welcomed him and his family by lighting rows of clay lamps to the city. Diwali is an occasion in honor of Rama's victory over Ravana; of Truth's victory over Evil.

However there are many other stories:
King Bali and Vamana Avatar: King Bali was a generous but ambitious ruler. Some of the Gods pleaded Vishnu to check King Bali's power. Vishnu came to earth in the form of a Vamana (dwarf) dressed as priest, approached King Bali and said "You are the ruler of the three worlds: the Earth, the world above the skies, and the underworld. Would you give me the space that I could cover with three strides?" King Bali laughed, agreeing that a dwarf could not cover much ground. At this point, the dwarf changed into Vishnu and his three strides covered the Earth, the Skies and the whole Universe. King Bali was send to the underworld, and Diwali is celebrated with some Hindus remembering King Bali.

Defeat of Narkasur by Lord Krishna: The demon Narkasura was a demon of filth, covered in dirt who would kidnap beautiful young women and force them to live with him, causing great unhappiness amongst the people of the world. Their cries for rescue were heard by Vishnu, who came in the form of 8th incarnation as Krishna. First, Krishna had to fight with a five-headed monster who guarded the demon's home. Narkasura hoped that his death might bring joy to others. Krishna granted his request and the women were freed. For Hindus, this story is a reminder that good can still come out of evil.

Krishna and the Mountain: In the village of Gokula people prayed to the God Indra because they believed that Indra sent the rains, which made their crops, grow. But Krishna came along and persuaded the people to worship the mountain Govardhan, because the mountain and the land around it were fertile. This did not please Indra who sent thunder and torrential rain down on the village. The people cried to Krishna to help, and he saved the villagers by lifting the top of the mountain with his finger. The offering of food to God on this day of Diwali is a reminder to Hindus of the importance of food and it is a time for being thankful to God for the bounty of nature.

Sikhs: When the sixth Guru, Guru Hargobind Ji was being released from his captivity at the city, Gwalior in autumn of 1619, he asked the Emperor if his fellow Rajput princes could also be released. Emperor agreed, provided each prince was “attached” to Guru Hargobind Ji. Since there were 52 princes, the Guru got a cape with 52 “tails” wherein each of the princes held onto each tail, thereby getting released with the Guru. To commemorate this, many diyas (oil lamps) were lit along the way to Harmandhir Sahib (Golden Temple). This day coincides with Diwali, and thus Bandi Chhor Divas ("Day of Liberation") and Diwali are celebrated on the same day.

Jains: Diwali is one of the most important festivals for the Jains who celebrate the Nirvana of Lord Mahavira who established the dharma followed by the Jains. Lord Mahavira was born as Vardhamana on Chaitra Shukla 13th in the Nata clan at Khattiya-kundapura, near Vaishali. He obtained Kevala Gyana on Vishakha Shukla 10 at the Jambhraka village on the banks of Rijukula river at the age of 42.

Gujaratis and Marwaris: For the business community, Diwali marks the worship of Goddess Lakshmi and also the beginning of the new financial year.

Bengalis: The celebration is to worship Goddess Kali or Durga who continued her "Vilaya Tandava" even after killing demon Mahishasura.

Fireworks: Modern India has been celebrating Diwali with a huge amount of fireworks, so-much-so, that pollution caused by these fireworks has led them to be banned in metro cities like Delhi. But fireworks have been used to celebrate Diwali for many hundreds of years.