First celebration of Diwali
There is end number of reasons for Diwali celebrations. There is no appropriate evidence about when Diwali celebrations were started. Diwali has been celebrated for many years but no one knows the exact date.
It is a major festival not only for Hindus but also for Jains, Sikhs, and some Buddhist. For Hindus, it marks the arrival of King ram to his empire Ayodhya after being in the state of banishment for about 14 years.
Diwali was First Started after return of Lord RAMA
During that time he defeated demon king Ravana which symbolizes victory of inner light over spiritual hollowness, ego and greed.
It also marks the day when demon Narkasura and Demon Mahisasura were killed by Lord Krishna and Goddess Durga respectively.
For Jains, it is a day when nirvana was attained by the Lord Mahavira who is last Jain Tirthankaras whereas for Sikhs Diwali is related to their freedom struggle. While Guru Nanak was fasting, a king tried to feed him meat, he refused to consume it.
People protested against this and were gathered outside the palace with lanterns. Soon he was released by the king and this is the reason why it is celebrated by the Sikhs.
Other than these religious reasons Diwali is also commemorated to mark the last harvest before the arrival of winter which makes it a harvest festival.