(Avatara Vamana splashes Bali’s head, and sends him to the Patala. photo Wikipedia)
Onam is special. It’s a celebration mostly by Kerala people when everyone, irrespective of religion, age or anything, is involved in the festivities.
Onasadhya, the very tasty vegetarian meal with numerous dishes, served on fresh banana leaves, is unique. For children it’s the new clothes, fondly called ‘onapudava’ to look forward to.
Onam is the homecoming of the famous and gracious Asura king Mahabali, who was deposed from his kingdom in a ‘trick’ act in the rivalry between ‘devas’ the God’s favourites and the powerful ‘Asura’.
The Onam celebration include a procession with a decorated Mahabali walking with the crowd with his special umbrella.
Onam also remembers the abundance during this king’s rule. For many Malyalees, a pot belly signifies abundance and prosperity. So the person dressing up as Mahabali is big pot bellied.
But actually Mahabali is ‘Mahabalawaan’ the warrior king and should be sporting six packs and not the pot belly.
The story most attractive to me was told by a professor of Tezpur (now again Sonitpur) in Assam. And now we have the Wikipedia details confirming the story when you search for Mahabali and Banasura.
“Banasura is believed to have ruled the present-day central Assam with his capital at Sonitpur (present-day Tezpur, Assam). According to the legend, Banasura was an aboriginal king. Some other sources say that since Banasura, son of Asura King Mahabali who is believed to be central character in mythology and culture of Kerala, inherits his kingdom from his father and is believed to have ruled from Kerala. There is a hill named “Banasura hill”  and a dam, “Banasura Sagar Dam” dedicated to the memory of their great ruler’s son Bana.”
“The genealogy of Banasura is as follows:
• Brahma’s son was Marichi
• Marichi’s son was Kashyapa,
• Kashyapa’s sons were Hiranyakashipu and Hiranyaksha,
• Hiranyakashipu’s son was Prahlada,
• Prahlada’s son was Virochana,
• Virochana’s son was Bali,
• Bali’s son was Banasura”.
The professor told me that, Assam at that time was known as ‘Pathala’ the unknown place. When Vamana Muni didn’t have the land to measure the third step, the proud Mahabali asked him to step on his head and the muni got him banished to save the worried devas.
The famous theatre in Tezpur is ‘Ban’s Theatre. Original Assamese women attire ‘Mekhla Chadhor’ is similar to ‘Ottamund’ of traditional Kerala women. Both Malayalee and Assamese are rice eaters with even have puttu as uniquely special. (Cooking method only is different).
I am not seeing this story getting mentioned in the many write ups on Onam, and I am happy to see the details on Wikipedia.