But of course, one can be a bit more comprehensive:
Lead me from the
untruth to the truth.
(Brhadaranyaka Upanishad — I.iii.28)
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.
there are many other stories:
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.
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: Sikhs celebrate Diwali as the return of the sixth Guru, Guru Hargobind Ji from the captivity of the city, Gwalior. To commemorate his undying love for Sikhism, the town people lit the way to Harmandhir Sahib (Golden Temple), in his honour.
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.