Deepawali according some beliefs coronation of Vikramaditya took place day of Rama and Krishna

Significance of Deepawali Why is Diwali celebrated?

 

The importance of Diwali is from Lord Shri Ram 

In the last phase of exile, Lord Rama was welcomed by lighting a lamp in Ayodhya in the joy of victory over Lankapati Ravana and returning home after 14 years.

One day before Deepawali, on the day of Naraka Chaturdashi 

Lord Krishna killed Naraka Sur in his childhood and liberated sixteen thousand girls of God Manav and Gandharva who were imprisoned near him. Those who returned to their respective homes, were later married to Lord Krishna with sixteen thousand girls.

It is believed that on this day only on the day of Kartik Amavasya, Lord Krishna returned to Baikunth from the dead world.

According to some other beliefs

The coronation of Vikramaditya took place on the day of Deepawali and Vikram Sambat started from this year itself. 

It is said that Lord Vishnu was married to Goddess Lakshmi on the day of Diwali

The main purpose of the day of Diwali is the worship of Goddess Lakshmi, the god of wisdom, worship of Lord Ganesha, worship of Lord Kuber for splendor and wealth.