Hi Folks, here is the orignial story reason for celebrating Onam!
As everyone knows that Onam is celebrated for Mahabali King who lived. There is a myth behind this King. Here it goes:
As everyone knows that Vamanan Avatar is one of the Avatar of Vishnu. Mahabali is a philanthropist/ultruist (unlike selfish) in tamil "kodaivallal" whenever a person asks for anything he will never regret anything he will give that person whatever he asks for. At that Vamanan asked for 3 feet of land. For that Mahabali decided to give it. Vamanan took one feet as the sky, the another feet as the earth and when Vamanan asked for another feet for that Mahabali gave his head as the third land. Vamanan has become very much happy. On this day it is celebrated as a Onam. In this month of Onam it is considered that Mahabali king will visit one day to another of their house and make that family happy and happier. This is the Onam has started and in this day the boat race is conducted in the near by side river. After this incident there is a very famous phrase has formed in tamil "kallana kooda nambalam aanal kullana nambavey koodadhu". Meaning "you can believe anyone in this world but not the short person or the dwarf". Because Vamanan appears like a dwarf and shows the a very big and tallest person.
Please look into the images which I have attached along with this post!
-Charu