Story of Dhanteras
Once Goddess Lakshmi wanted to accompany Lord Vishnu to the earth during one of His visits. Lord Vishnu agreed to take Her with him provided She will not fall for earthly temptation and will not look in the southern direction.
But Goddess Lakshmi was tempted to look in the southern direction and when she got an opportunity she looked in the southern direction.
In southern direction she saw yellow mustard flowers and started dancing in the fields and decorated herself with the flowers. She fell for earthly temptations.
Further south she saw sugarcane fields and started enjoying the sugarcane juice.
Lord Vishnu soon found out that Goddess Lakshmi had violated the conditions and said that now you will need to spend 12 years as laborer in the field of the person to whom the sugarcane and flowers belonged.
With the arrival of Goddess Lakshmi as laborer the farmer became very prosperous. Twelve years passed and it was time for Goddess Lakshmi to return.
The farmer returned home happily realizing that it was Goddess Lakshmi who was staying in his house. He requested her not to leave. Goddess Lakshmi said that she cannot live on earth permanently and has to return. But she said that she will visit him on all Dhanteras day. She asked the farmer to keep the house clean and light an earthen lamp.
The farmer said as directed by Goddess Lakshmi and he became very prosperous and people who came to know about the incident started performing the same ritual on Dhanteras day.